What makes you hesitant to support your childs relationship with their birth parents?

We are in the first steps of the process to adopt my nephew from out of state. We will never try to hide anything from him after he becomes our son, and we have agreed to only tell him the best parts of his past, but letting his birth mom become too close is scary because we feel as though she may try to be the mom and negate our roles as his parents. This is not a voluntary separation for her, and she is my sister, it will be very difficult for her to understand that it's over. If she ever does make it all the way here (to Michigan from Arizona), on her own, it would be a miracle. It would also mean having to protect our family from family. It's touchy for relative adoption cases I suppose is what I'm trying to say.