Wish I had answers to give. I think what you SIL said is very good. Don't beg or ask, just state that you still love her and do not want a divorce. I think brief and matter of fact would work best.
I'm sorry you are having this difficult time. I am sending prayers and encouragement your way.
bambam
Me-BS 38 X-WS 36 Separated 11/15/2006 Filed for D 8/1/2007 Divorce Final 12/21/2007 S13, S13 (twins), D9 Married 13 Yrs Together 20 Yrs