1,462. Giving Kiddushin to a Prohibited Woman
Hilchos Ishus 4:13
If a married woman accepts kiddushin from another man in front of her husband, she is consecrated to the second man because a woman is believed when she tells her husband that he divorced her. There is a presumption that a woman would not behave this way in front of her husband if they were still married. However, if the second man consecrates her not in front of her husband, then marriage is not effected until she proves that she was divorced before she was consecrated. This is because it is possible that a woman might act that way when not in front of her husband.
Hilchos Ishus 4:14
If a man consecrates one of the secondary prohibited relationships, or a woman forbidden by a prohibition (lav) or as the consequence of an obligation (asei), then marriage is wholly effected. The same is true if a man consecrates a woman who was co-wives with another woman whom he married through yibum (levirate marriage). However, if someone other than a deceased man’s brother consecrates his widow (who is awaiting yibum), then there is a doubt as to whether or not marriage is effected. The Sages were unclear whether a woman awaiting yibum is like a relationship prohibited by a lav or like one of the inherently-prohibited relationships. In all of the above scenarios, even though the man is not permitted to consummate the marriage, he must nevertheless dissolve the relationship by giving the woman a get.