DH and I were recently faced with a possible unplanned pregnancy. DH is adamant he doesn't want any more children, I'd like one more but not yet.
Anyway, I was showing symptoms of being pregnant, and my AF was late (and is very regular due to being on the pill).
Before I took a test, DH and I discussed what would happen if I were pregnant. He told me he wouldn't be happy about it, but we would keep it as he wouldn't want me to terminate a child of ours.
I didn't end up being pregnant, but he certainly would be supportive through it all.

What a horrible situation for your sister to be in! I hope there is some resolve that she is happy and comfortable with.