Simchadig

From everything I have read in seforim or been told personally by my Rav, a Ger does not have to disown his or her pre-conversion family. In fact, when a non-Jewish family member dies, the ger may sit shiva for them (but is in no way halachically obligated to do so). There is no prohibition against yichud or negiah with one’s non-Jewish siblings, parents, or grandparents.

The pre-conversion family is no longer one’s halachic family, but that does not mean that a ger could marry his mother or sister. I was told that such a relationship is forbidden on the grounds that we should not be able to say that the Torah sanctions immorality.