Sorry you're in this situation -- the same thing happened to me where both my sister and sister-in-law had their marriages end the same time mine did. In my sister's case, she reconnected with her high school sweetheart, had an affair with him, and is now divorced and has been married to OM for 5+ years so he's now "H".
WRT your sister, there is likely no telling her anything. Most WW will seek people who will support and validate them. If you don't support her, chances are she'll just stop confiding in you and keep doing what she's doing anyway. Therefore, your best bet with her is listen and validate her feelings. Her feelings are her feelings, there's no right and wrong in that, they just are what they are. You don't have to agree with her actions or her decisions in order to support her.
I would reach out to your BIL and tell him not to pursue, and to give your sister more space than she's asking for. Tell him to go the other way and observe the impact it has on her.
Initially she'll be relieved and will put him out of mind. Eventually the shine will come off of OM and she'll wonder where BIL went and why he was willing to move on so easily and then she will begin to doubt what she's doing.
As long as he's around and pursuing, all she's going to focus on is getting away from him.
By seeing what works for BIL, you can see what will work for you.
Good luck!
Acc
Married 18, Together 20, Now Divorced M: 48, W: 50, D: 18, S: 16, D: 12 Bomb Dropped (EA, D): 7/13/11 Start Reconcile: 8/15/11 Bomb Dropped (EA, D): 5/1/2014 (Divorced) In a New Relationship: 3/2015
