Is it halachically permissible for a man to marry the daughter of his mother's husband from a previous marriage?
Marriage between a step-brother and a step-sister is permitted, and such is accepted practice. The Chafetz Chaim, Rabbi Yisrael Meir Kagan, married his mother's husband's daughter. I personally know a couple like this who have been happily wed for the past 30 years.
According to Tzava'at Rabbi Yehudah Hechasid step-siblings should not marry. However, this ruling seems not to have been accepted even by those who generally adhere to the other rulings of Tzava'at Rabbi Yehudah Hechasid.
- Shulchan Aruch Even Haezer 15:11
- Sefer Shmirat Haguf v'Hanefesh 178