Grandparents & Grandchildren
When Yaakov blessed Yosef and his children just prior to his death, he began by declaring that "Efraim and Menashe will be to me just as Reuven and Shimon", as if to say, "Your children are like my children". It is also noted that Yitro's daughters addressed and referred to their grandfather as "father". The Talmud rules that in many instances "bnei banim harei hem k'banim" - grandchildren are considered to be as one's actual children. We are even taught that some grandparents have a tendency to prefer their grandchildren over their own children. Nevertheless, on the verse "And Yisrael….offered sacrifices to the God of his father Yitzchak", the Midrash notes that, while a person must honor his grandfather, honoring his father comes first. The topic of grandparents occupies a prominent place in halachic literature, especially as it relates to the responsibilities that grandparents and grandchildren have to one another.
An interesting application of halachic principles which pertain to the relationship between grandparents and grandchildren is the mitzva of "pru u'rvu", of having children. For example, in the tragic event that all of one's children passed away before having children of their own, one would not have fulfilled the mitzva of "pru u'rvu". However, if one's children had children of their own before they died, one is considered to have fulfilled the mitzva of "pru u'rvu". This is because the mitzva of "pru u'rvu" is fulfilled through one's offspring, not necessarily through one's actual children. Similarly, if a man has no living children but he does have living grandchildren, he is not considered to be childless, and therefore yibum is not performed if he dies before his wife. A grandparent may recite the "shehecheyanu" blessing upon the birth of a grandchild should he so desire.
It is a matter of dispute whether or not there is an obligation to honor grandparents as one must honor parents. While most authorities insist that the mitzva of "kibbud av va'em" does indeed apply to grandparents, others strongly disagree. Indeed, the Talmud itself seems to imply that there is no specific mitzva to honor grandparents. When Rav Acha asked his grandson Yaakov to bring him water, he responded, "What? Am I your son that I am required to honor you?" Some suggest that the only reason one is truly obligated to honor grandparents is because doing so is pleasing to one's parents. There is also a view that the mitzva of honoring grandparents is rabbinic in origin rather than directly from the Torah. In any event, pleasing one's parents in any way, including showing honor to their parents, is certainly a component of kibbud av va'em. There is an opinion that the mitzva to honor grandparents applies only as long as one's parents are alive.
Make no mistake; even according to the opinion that the mitzva of "kibbud av va'em" does not apply to grandparents, one is nevertheless still required to honor one's grandparents under the clause of "vehadarta pnei zaken", the mitzva to honor all elders. We see a similar idea regarding one who curses a parent which is an act that is subject to the death penalty. Cursing a grandparent, however, is like cursing any other Jew for which the punishment is not as severe.
It seems that a grandfather has an obligation to ensure that his grandchildren receive a proper Torah education in the event that the parents are not able to do so. In fact, in the event that parents cannot afford to pay the tuition fees for their children's Torah education, a beit din can force the grandparents to do so should they have the means. However, a grandparent is not required to tend to the marriage-related expenses of a grandchild.
A grandchild is permitted to recite kaddish for a deceased grandparent. However, in a community where the custom is that kaddish is recited by only one person at a time, those reciting Kaddish for a parent take precedence. After such individuals have recited at least one kaddish, any remaining kaddishes may be allotted to those saying kaddish for a grandparent. It is actually considered meritorious to recite Kaddish for one's grandparents if there is no one else who is able to do so. However, one should never recite a mourner's kaddish, for any reason, while one's parents are alive unless explicit permission has been given by them to do so.
There is a well known Torah law that when one accidentally kills another person, the accidental murderer must flee to a city of refuge. In the event that the murderer does not relocate to one of these cities, the children or other family members of the deceased are permitted to "redeem the blood" and kill the murderer. In the event that it was a father who accidentally murdered his son, the grandson becomes the designated "redeemer of the blood" and would be permitted to kill his grandfather if he does not flee to one of the cities of refuge. In contrast, the brother of one who was accidentally killed by their father would not be permitted to kill the father under the "redeemer of blood" clause, because the mitzva of "kibbud av va'em" takes precedence.
There is a case discussed in halachic literature of an orphan who is about to have his brit mila and the grandfather is the designated mohel. The question was asked as to who should recite the blessing of "l'hachniso bevrito shel Avraham Avinu", as many authorities are of the opinion that in such a situation it is the sandek who should recite the blessing. Although it is true that in such situations it is the sandek who would normally recite the blessing, Rabbi Akiva Eiger rules that in such a case it is the grandfather who should recite it even though he is the mohel and not the sandek. This is because he is inherently more connected to the baby than anyone else and he is also responsible for the child's Torah education. Similarly, in the event that a Bar Mitzva boy's father is no longer alive, the "baruch sheptarani" blessing may be recited by a grandfather.
One whose father is no longer alive inherits his grandfather's estate just as his father would have. A person whose father is not Jewish is called to the Torah using a grandfather's name (on his mother's side, of course) or as the son of "Avraham Avinu". Grandchildren should recite the blessing of "sheasa nais" when visiting a place where a miracle occurred to a grandparent. It is recommended to consider using the name of a departed righteous ancestor when naming children. Indeed, one who feels that one's parents or grandparents were righteous should consider naming one's children after them. Nieces and nephews are occasionally equated with grandchildren in Talmudic literature.
One of the "Six Remembrances" that a Jew is always required to remember is the revelation at Sinai. There is a special mitzva upon grandparents to teach this to their grandchildren, as well. As the Torah states: "V'hodatem l'vanecha v'livnei vanecha yom asher amad'ta lifnei Hashem Elokecha b'chorev", - And you shall tell your children and grandchildren about the day you stood before Hashem your God at Chorev." Indeed, it is taught that the experience of a grandparent teaching Torah to his grandchild is comparable to having been present at Mount Sinai when Moshe Rabbeinu taught the Torah to the Jewish people.
 Bereishit 48:5.
 Rashi, Shemot 2:17.
 Yevamot 62b.
 Zohar, Vayechi.
 Bereishit 46:1.
 Rashi, Bereishit 46:1; Bereishit Rabba 94.
 EH 1:6.
 Yevamot 70a.
 Devar Chevron 2:203.
 Darkei Moshe, YD 240:6; Rema, YD 240:24; Sheilat Ya'avetz 2:129; Minchat Elazar 3:33.
 Biur Hagra, YD 240:24; Chatam Sofer, YD 345; Rema, YD 240:24.
 Sota 49a.
 Tuv Tam V'daat 1:213.
 Tuv Tam V'daat 1:213.
 Vayikra 19:32.
 Rambam, Hilchot Mamrim 5:3.
 Kiddushin 30a; Kesef Mishna, Hilchot Talmud Torah 1:3; Shach, YD 245:1. see Shevut Yaakov 1:173 regarding great-grandchildren.
 Shulchan Aruch Harav, Hilchot Talmud Torah 1:8.
 Rema, YD 240:24; Knesset Yechezkel 240.
 Rema, OC 132:2; Pitchei Teshuva, Y.D. 376:4; Kaf Hachaim, OC 55:27-28; Yabia Omer, YD 3:26; Devar Chevron 2:56,58.
 Bamidbar 35:11-24.
 Rambam, Hilchot Rotzeiach 1:3.
 Teshuvot Rabbi Akiva Eiger 42.
 Maharsham 8:33, cited in Piskei Teshuvot 225:4.
 Pri Megadim, OC 225:20.
 Bereishit Rabba 37:7.
 Aguda, Shabbat 81; Menachot 35a.
 Rashbam, Bava Batra 108a; Mordechai, Gittin 431; Mordechai, Ketubot 235; Teshuvot Harosh 26:22.
 Devarim 4:9-10.