How about a tape recorder? Keep one ready in your pocket, and if you think he is going to say something she doesn’t know about, turn it on. Play the tape for her.
I’m not a lawyer, and this might be illegal, it may not even work anyway(if she doesn’t want to believe it, she won’t).
Usually, I end up going to their bloody wedding.
Mind your own business
You could always try 10 Stupid Things Women Do to Mess Up Their Lives by Dr. Laura.
Yes, I know, she’s not the most revered person on the board, but she does have some interesting insights.
"What do you do when a loved one is dating the wrong person? "
If I think it’s the wrong person, I don’t do nothing.
If they think that it’s the wrong person & it’s been going on for a year, then I ask them why didn’t they do something about it a long time ago?
The harder you try to pry them apart, the closer they will get.
You don’t say how old your sister is but she must be old enough to know better.
All I can suggest is be there when its over. And don’t say “I told you so”
I also think sometimes there is a fear of another failed relationship. The person continues in their current relationship regardless of whether or not the relationship is a good one just because they don’t want yet another failed relationship under their belt.