Don't talk to me about mothers! Mine is a strange one. She buys toys for ds in charity shops, and they always have something missing! It really gets my dh's back up, although ds loves them as he is too young to worry about that sort of thing. She also spends about £40 on me and dh at Xmas, but only spent about half that on ds' presents. I always tell her that I would rather that she spent the money on ds, but she never does.
She never used to kiss ds when he was younger, and it got to the point where the difference between her and my dad was blatently obvious, as my dad rolls around with ds, and always kisses him.
The final straw (seems petty, but it stems from my childhood relationship with her, which is too long to go into) was when she told me that she and dad were going to be on (another) holiday on my ds' next birthday (his 2nd, they were also on holiday when he was born!). I got all huffy, and eventually told her, when pushed, that we didn't have a mother and daughter relationship (more like friends, as we have got over the past, and do see a lot of each other), and that I was always watching her with ds, to see if she loves him, as I don't want him to get hurt. Anyway, to cut a long story short, she has since been more affectionate with him, and it is obvious that he loves her.
Titchy, don't let her emotionally blackmail you. Do what is best for your family (in that, I mean you, your dh and your children), and be happy. If she can't accept that, then it is her problem. It is not unreasonable to see the other parents, especially when it sounds like you will have loads of fun there, AND you have seen her for the previous two years.
Good luck - you can choose your friends, but not your family (unfortunately )