Back again, Netwife!
Wow! "A fine mess," indeed! I'm so very sorry! It's hard enough, I'm sure, to be pushed away by one person. But when a number of family members are involved, it must be that much more painful. I can barely imagine it!
I get your suspicion that your mother and sister are avoiding you cuz they resent your moving away/being more independent. Is your mom the kind who had trouble letting go? Then this may have something to do with it and why she tends to side with your sister who, I take it, has remained closer to her. I would suggest that it may be merely the distance that has cut down on your communication - and I'm certain that's part of it - but given the rest of your post, there seems to me more going on.
In fact, it's also possible that the strain between you and them is based solely on the issues surrounding your ex. I take it there has been a lot of tension and argument over their continuing to be friendly with him. Am I right? If so, perhaps it's better if you don't have that much contact with each other, for now.
It seems to me that DD (dear daughter) has been caught in the middle of all this. Unfortunately, when you told her about your ex and your sister, that may have been more than she could bear. Still, it seems that cutting you off completely was a little dramatic. But chances are, she's just taking a TO (time out), a break/breather from the relationship with you so that she can sort out her thoughts and feelings. Has she ever done this before?
If she doesn't want any contact, right now, it doesn't surprise me that she hasn't answered your letters, etc. That would be "contact" and she's not having any. You might want to back off, for a while, and give her some space - give her a chance to miss you and so forth. I don't know... your call... If you do write to her again, you may need to apologize for speaking against her dad/giving her TMI (too much information), that is if you haven't done that already.
Meanwhile, I get why it hurt you that sister and niece visited your mother and DD, w/o contacting you. But again, given the strain on all sides, I'm afraid I understand why. I'm so deeply sorry, but unless and until these issues are resolved, I think you'll have to expect this to happen, now and then.
Anyhow, I wouldn't be too quick to run to court for visitation with your GC, especially since DD may just be taking a temporary break. If she just needs some time and you haul her into court, that may further damage your relationship with her. You may not care, as long as you get visitation with your GC. But there's no guarantee you'll win. And if you don't, the fact that you went the legal route may prompt DD to turn "temporary" into "permanent." Also, honestly, as you say, there's no guarantee the kids haven't been "turned against" you or, more likely, just told their mom's POV.
I hope all this will work itself out, eventually. But I'm afraid it's going to take time, lots of time. More (((hugs)))