How to get my sister to not have a child with this man?

I know you love your sister, but what is this about? Saving her or saving you? If it is about controlling someone else to save you then butt out. If you feel self righteous enough to want to control your sister's life even though she is 30, at what point will you feel she is old enough and has a right to make her own decisions and live with the results. I think you may have good intentions, but take a step back and recognize you are way out of line with this approach.