Namsnanny, maybe Idk whereof I speak because I'm not estranged at the moment (who knows what the future holds?) though I do have friends who are. But, imo, your reaction is good, if only because you know you tried - and, in later years, you can tell your gc that you tried. Sorry you're feeling down today and sending hugs!
Dontask me, I'm glad you're coming to terms with your situation and have a "mission," as you call it, that helps you feel better. I don't want to raise false hopes, but isn't it possible that your gc will reach out to you when they are old enough to do so on their own? I know it doesn't necessarily happen, but Iv heard of it, even on here, I think. So you may see them again, even if, sadly, it's not for a long, long time.
And that's why, Aqua, I don't think gd will "forget all about" you. The fact that she expresses missing you shows you are an important person in her life. Her parents can loosen the ties , but they can't erase all the fond memories. Again, I don't want to raise false hopes, but I believe she will reach out to you again someday when she can do things on her own.
Oh, and I want to amend something I said earlier. I don't think as and dil are calling sister "grandma" out of respect for xh. I imagine it's for gd's sake. How can her great-aunt be married to grampa? Confusing for a child and yet a very messy situation to explain. I'm not crazy about the way the parents are handling it, but I suspect they don't want to have to tell her that people do what your xh and sister did. So they'd rather she not know that sister was ever that, your sister. I don't like this - secrets and lies - not a very good plan, imo. But it's a complicated situation that sister and xh created. And I'm not sure if there's any "good" way to handle it. I just hope that realizing that they might be calling sister "grandma" for gd's sake might make it easier for you to bear.
If that's what they're doing, I get why they don't want you to explain that sister is, in fact, your sister. But I don't think they should have expected you to know that. We're none of us mind readers, after all. If you get to talk with as, and if you think Iv hit on the true reason for this, perhaps you can tell him that you understand now and that you won't mention anything like that again? I know you didn't ask for advice, but I just can't help that you might be able to resume a relationship w/ gd if the parents see they can trust you to follow their t & c.
I don't get why they told gd you aren't "family" though. Why can't there be two gms from as' side - a gm and a sgm (stepgrandmother)? Lots of families have that situation. Or did they mean you're not "immediate family," as I think agnurse mentioned? They may have simply meant you're not part of the core family that includes just them and gd. If you get a chance, I would ask as what "not family" means just as a question, not a complaint.
I know some posters didn't find agnurse' distinguishing between immediate and extended family helpful. But if that's all as and dil meant, then perhaps it's not as bad as it sounded when gd said it. If they meant more than that, then I don't get it and I'm very sorry.