ooops - I am also adopted and my bparents married after I was adopted and I have absis and bbro. Oddly enough, my bmom is also partially deaf (and my adoptive parents ahve both died).
My bmom is, er, a bit bonkeers, frankly. I have built up a reasonably solid relationship with my bsis but we have an understansing that we don't discuss our parents - the relationship is just about us and our lives. I found this hard for a while (who else could I talk to that knew our parents?) but really it makes sense.
IMO, You have no obligation to your bmom other than what arises from the relationshiopyou have had as adults plus whatever benefit you feel arises from her wanting contact with you as an adult. It is possible to seperate the two relationships - your bmom and bsis. But also - the realtionship between your bsis and bmom is their business - you do not need to be involved.