2,170. Severing Previous Bonds

Hilchos Issurei Biah 14:11

When a non-Jew converts or a servant is freed, he becomes like a newborn infant. Any relatives he previously had are no longer legally considered his relatives. If both he and his relatives convert, he is not liable for engaging in intimate relations with any of them.

Hilchos Issurei Biah 14:12

Under Biblical law, a convert is permitted to marry his mother or his sister from the same mother after they convert. The Sages prohibited this so that converts shouldn’t think that they transitioned from a higher to a lower level of holiness because a formerly-prohibited relationship is now permitted. Similarly, if a convert is intimate with his mother or sister when they haven’t converted, it’s as if he was intimate with a woman to whom he’s not related.