Faith,
I can't give direct advice b/c I have never had this issue. But one thing I think causes problems among some of the issues in acceptance of a step child is the adult may see behavior that they feel is unacceptable or annoying. And in some cases it may be and the parent is blind to it. But what I've figured out is that the step parent has the annoyances (extreeeeeemly apparent in older children) but do not have the bonding the bio parent had. The joyous memories of a first step to block the rude comment of a 13yo boy or the first momma or daddy to blur the eyeroll of a 11 yo girl. There has to be a balance. The outsider has to find their own balance if there is to be a positive relationship between adult and child.