NYBabe:
If you already know in your heart and in your head that everything's OK, and that there is no problem with you dating your former cheating partner and ex-brother-in-law, why are you cluttering the internet with your question?
If you asked the question to see what others would think about your proposed dating, well, you've found out what others would think. You oughtn't to complain about getting honest opinions when you asked people to give them to you.
If you asked the question hoping to be encouraged to do something you suspected to be fundamentally wrong (not to mention just a bit sick and perhaps evil) then you failed to get your encouragement, but you had your suspicion confirmed, which might be a much better result than encouragement would have been.
Anyway...
I do disagree with the post that said if you go ahead with your lover/cheater/ex-BIL you'll be "burning your bridges." I think you burned your bridges years ago when you cheated on your husband with his brother.
If you stay with that loser, you're not going to burn a bridge, you're going to set the whole damned river on fire.
When in trouble,
Or in doubt,
Run in circles,
Scream and shout.