You might relate this story to her. I used to rent a room in the home of a couple both of whom were divorced and re-married. Maybe they had annulments, maybe not, I don’t know. But I remember a conversation I had with the husband one night when his wife was out. He said, “I love Diana, but I wish I’d had the sense to figure out what went wrong with my first marriage before I left. It turns out that I was a jerk who didn’t understand what it meant to be married when I got into it. I had to learn that this time. If I’d learned that the first time, I wouldn’t have a ex-wife and two families to support, our kids would have had a stable home to come to, and we’d have saved everyone a lot of grief.”
You can divorce a spouse, but annulment or not… an ex is forever. Second marriages have a worse chance of succeeding than first marriages because so many people turn around and get into another version of the first marriage that didn’t work.
Your BIL and SIL owe it to themselves to figure out what is wrong and to be certain it cannot be fixed before it is too late. Otherwise, there will be a lot of needless suffering, and it won’t just be their suffering, either. Not doing that work is unfair, both to themselves and everyone else they’re hurting now and will be hurting if this goes on.
Just a thought.