Sounds like she is having issues with other kids at school that she doesn't want to tell you about, or it could be the teacher and her don't get along... lots of things can be the problem and trying to get some kids to tell you what it is can be next to impossible... It might be easiest if you could get an older kid to talk to her and find out why she hates school so much.. you mentioned she got on well with some older cousins, one of them might be able to find out the answers for you. Often a kid will open up much more with another kid while say nothing to an adult or parent.
