It would have caused issues for me, because lots of the harms and resentments on my inventories were about family, so it would have put her in a difficult situation.
Also, there were times when I griped and complained about my partner and my sponsor (rightly) gave me short shrift, and told me to focus on my own inventory if I wanted to get well. Times when he behaved like an arse and I ranted and raged, and my sponsor quietly said the serenity prayer to me and reminded me of the importance of acceptance. Times when his drinking infuriated me and I blurted out all the stuff that HE should do to turn HIS drinking round, and she told me to keep taking my own advise. Now, I could take that from her in every instance as she did not know my partner and there is no way that she isn't neutral. But if she was linked to my partner as in a family tie? Im not sure I'd have taken that valuable advise from her then.
Whether any of that is likely to be an issue for you, only you and she have an inkling of.
Thing is, your SIL is always likely to be in your support network anyway, so by asking her to do it you aren't necessarily building up that network. Do you think that there is a possibility that this idea is more about avoiding that horrible prospect of asking someone else to help you? Of having to make a decision about who to ask?
Have you asked her what she thinks of the idea of sponsoring you? I have made a policy (after a horrid experience) to not sponsor very close friends, and I think I'd say the same if it were ever a family member as well, unless there really were no one else to do it who would be a good sponsor. The blurring of boundaries is inevitable and can easily cause problems. Again, that's just me, and it's something I learned through experience.