My SIL went loco after our baby died. She was great for about the first 4 weeks then tried to make much of our loss about her. She really had no understanding at all. Fortunately for us she did not split us with her behaviour like your SIL is doing. My hubby is clever enough to see her for what she actually is.
I honestly think everyone "copes" with the loss of a baby differently so she might actually be behaving badly because she also feels cheated that Char-Leigh is not here to play with her baby.
This in no way makes her behaviour OK but it might be an explaination.
Many husbands don't want to go to counselling. They think it's never going to help them and they feel like they are giving in to the grief instead of fighting it by accepting help. I would see if you can get your hubby to come see a counsellor with you. Tell him it will help you if he comes along with you. Sids are really good for this. We saw a counsellor for about 4 months together then on our own. I still see someone but hubby doesn't feel the need anymore.
Good luck,
Kate