Hello Jonathan: I see this is your first post here on PC. So... welcome to Psych Central. We here on PC are not able to give legal advice which, it seems to me, is what is called for here. It's unclear to me what country your sister is in. I see you list yourself as being in the U.S. But it sounds as though she is, perhaps, in Ireland?

Under any circumstances, at least from my lay-person's perspective, it sounds to me as though the answer here may be for someone in the family to seek legal guardianship over your sister in accordance with whatever the relevant laws in in the state or country in which she resides. If her mental health status is not such that she would meet the criteria for guardianship, then I think what may be left you & her parents is to simply step back & allow circumstances to unfold as they will. As things stand now your sister is an adult. And she has the right to make her own decisions... bad as well as good. If your sister does end up bringing her pregnancy to term, perhaps adoption might be something to consider, if your sister is willing. My best wishes to you both.