I have a good relationship with my stepson, though they do live some distance away so we don’t see them very often. I have been part of his life since he was about 10, and he has no contact with his mother. Yesterday he differentiates between me and my husband and it is made quite clear to their daughter that although my husband is her Grandad, I am Granny Cabbie, a step below, kept at a distance. There is no ill feeling, it is just the pecking order my step son has chosen. I am glad for my husband that he has a grandchild, as I have four, plus one step grandchild. I hadn’t planned to treat her any differently, but that’s the way it is.
I hope it works out for the OP.