You two obviously have different ideas of what is age appropriate for the girls.
At 17, I had graduated high school and gone away to college. Obviously I didn't need a baby sitter. And if my parents had suggested that I couldn't be home alone I would have laughed since I was living on my own and paying for college mostly by myself.
Some people are more mature than others though, so that should be taken into consideration.
XW cancels her plans to take them because she believes them incapable of taking responsibility for themselves. She also likes being the martyr I think. The fact that she believes she has to do this because you won't cancel plans to be with them just increases her resentment of you and solidifies her belief that you are not a cautious enough or involved enough parent. It's a viscous cycle.
But having someone check on them, taking them with, or even calling regularly to check on them are all viable options that should be calmly suggested IMHO.
Don't let this get to you. It's the same old, same old. Not worth getting worked up over.
Michelle - Proud DR Rockette S: 28JUL07, D'd: 29OCT09 http://tinyurl.com/27j9qo2