Sounds like she feels threated by you? unless she feels she has to fill your shoes, and I think maybe the reason she is only inviting your 11 year old is because she feels this child does except her, and she knows the other two don't. I have been a step parent twice and each marriage w/stepchildren I ran into the X-wife problems you are trying to deal with now and frankly to be honest with you I would not want to take my stepchildren that didn't want to be around me on my vacation, sorry to put this out there, but sometimes the stepchild can make it difficult for the new wife, and for some reason it is reversed when it's a stepfather? and it also works the other way as well?? I know this is difficult but what I did was kill her with kindness...
good luck...
Blessings