I am a step-grandmother, like many others. I have no contact with one of the families, (their choice.) One of my late DH's DDs has two GC and they are lovely — thoroughly grown up, and we get on well. I confess that when my only DD had my only GC, that relationship surprised me by being so much more intense.
My closest friend has five GC, four adopted from one family, and one the (surprise) child of the DS. She tries to treat them the same but says she can't help the relationship being different. The children themselves treat her in a different way. She tries not to let the adopted children feel that the new arrival is somehow more special.
The survey could have included questions about this, because it does seem to affect the relationship, however hard we try not to let it.