Hmm. Just wondering - what if she became involved with another man in addition to your husband, got pregnant, wasn't sure who the father was, and decided to have the child? Would you insist on knowing the paternity? Could you live with the uncertainty of not knowing? If it turned out the other guy was the father, would you be okay and accepting, maybe even play auntie, but if it was your husband's, would you really shun the child?
I am just curious. Anyway, I believe you mentioned that she's been thinking about the possibility - but has she clearly stated what she wants?