issues surrounding 'birth' mother/biological mother aside, I can partly see where the OPis coming from.
My dh's ex is still 'in' with his family, to the point where all my inlaws refer to her as their sil, dil, whatever, in my place (ie, I am not their sil/dil/whatever, because she is)
I am not at all bothered by her still being in contact with my inlaws, but it does annoy me that I am denied by them because they think they must have loyalty to her.
My bils have both said (on separate occasions) that their 'real' nephews/nieces are my stepchildren, and they do not count my children as relations.
which is just plain odd really.
and yes, it rankles slightly (only slightly, as I am not sure I want my children having much contact with people who hold such bizarre views, tbh) that there is a whole side of the family that do not accept my (and dh's) children.