S.R.
Being the evil step-mom in my house, I probably have a slightly different opinion. I must say first of all...I agree if she is sending u nasty e-mails just don't open them. Talk directly to your ex if you can, but if their house is like mine, that might not work. See, my hubby 'listens' to what is going on, then Forgets, and problems arrise. As for her being involved with the children and their activites...I would try to look at it like this, obviously she loves your ex and YOUR children which you should be entirely greatful for. A lawyer won't help; based on what you said it "appears" she is trying to be a good parent (a step parent is still a parent to those children when they are in her house).
Being a parent is hard work and being a step-parent is even harder. I say just keeping your girls' mother and try to ignore her when you can. Talk to your ex if you can about the e-mails. And don't let yourself fade or be pushed into the background at events or meetings (which I think is great that you and your ex can attend together...that will help your kids out greatly now and in the future). When she starts to pick a fight with you, pick a statement that you can use all the time, without appearing to 'back down'. Like if she complains about something "have **** call me"
Best of Luck