My SIL is the same way - my MIL and FIL have my niece and nephew all the time and she really prefers it that way. She just doesn't want to take care of them. She "wasn't able to take off work" to take my niece to preschool screening or attend my nephew's kindergarten graduation, but she had no problem taking time off the morning after she went to a concert because she was tired or for the two separate week-long vacations she went on with her boyfriend.
Of course, I blame my in-laws for all of this because she has always had everything handed to her and they do nothing but make excuses about why she acts the way she does. Some day these little children will realize that she is never there at the important moments (or most of the ordinary moments, either). I feel really bad for them. They both prefer to be at our home playing with our children, eating meals with us, and staying the night. They know there is a big difference between their home and ours. I don't think there is anything wrong with asking grandparents to watch their grandchildren occasionally; most grandparents relish time with them. But when it is day in, day out just because parents don't feel like parenting, then it becomes a problem.
