J.F.
I agree with everyone else. You do all of the legwork and decide if YOU can really do it. I had a SIL talk about doing this for me, but it wasn't a real offer and that really stayed with me. I appreciated that she had thought about it but it also hurt that she wasn't that serious and got me thinking about it a possibility that wouldn't happen.
You also mentioned that you just had your baby 9 weeks ago. Make sure it isn't your hormones talking here and give it some time before you make a final decision. :)
It takes a very special person to even consider doing this for someone. I think if you and your husband really looked at every angle and kept very positive communication open between you and your SIL it really could work and be a VERY positive experience for everyone.
Best wishes for you and your SIL.