I have functionally lived in a poly family for the last 19 years. I didn't know it was called poly and it wasn't always SEXUAL.
My best friend whom I call my sister and I raised my oldest daughter for the 1st 6 years. My daughter calls her "auntie" but their relationship is that of mother and daughter just as our relationship is.
A variety of boyfriends (and my daughters father) were in and out of our lives and all participated in some way-but Em and I were primary in the decision making process (obviously legally it was just me, but functionally it was equal).
I married when my daughter was 6 and he had a 2 year old boy. We had a son together. Em lived with us and continued to be a primary caretaker of our children (neither of us was or is sexually active with her and she lives with us now as well). She held more of a true auntie roll with my stepson, but with the new baby she was just as much a second mom as she had been with my oldest.
Years go by-our youngest is 7 and we had another little girl, she is biologically mine and C's but she is FUNCTIONALLY Maca's and mine. Maca IS daddy and I am mommy. Em is auntie and her roll is very much a typical auntie-not a mother role. C is an uncle roll, but she (as with all the kids except my oldest) call him by his given name with a "IE" added to the end. The oldest did always call him her uncle.
He has pretty close to equal say/so in the kids lives in day to day life. But MAJOR decisions remain only Maca's and mine.
Em also has pretty close to equal say/so in the kids lives in day to day life. But MAJOR decisions remain solely Maca and mine.
SO-can it work? Yes it can. But you need to sit down and discuss with your husband first WHAT is it EACH of you expect to happen and then find the ageed upon middle ground. After you two are on the same page you need to talk to her and find out what SHE is hoping her role will be with the kid(s). Try to take a serious look at her desires to see if they will benefit you. If they will-great. If not-then it's time to set ground rules.
By the way-you likely will have to do the same thing with grandparents as well-so you may as well use this as practice!