How do you stop a grown adult from doing what they want to do? You can't.

I also had a sister who married someone who ended up abusing her. It didn't happen before the marriage but my parents had reservations from the start.

Fortunately, she got out.....went on to marry someone else and have a child, and is happy (and safe) to this day.

But it scarred her emotionally. Even though she got out. I think it's hard to imagine the low self-esteem somoene has in this type of situation. They think they don't deserve anything better.

I think if I were you I might keep trying to get her into counseling. It's not too late. You don't have to support a relationship that you don't believe is safe for your sister. You can still be there for her, just in another way.

that's my take on it. best of luck, I know this is difficult.