Dd is 11 & is usually a loving & friendly girl.. Deffo not a spoiled brat.. But she really dislikes her dads dp.. They've been together nearly 2 yrs & lived together for 6 months. She is 15 years his junior.
No major battles just little things the gf has said & done that have upset dd. ie: insulting me & my friends in front of dd .. Calling my sister fat. Getting her dad to change their plans to suit herself. I think generally dd feels gf is 'taking her dad away'!
She is a health nut & doesn't allow my dd to eat mcDonalds or chocolate while she's with them & often comments on people's weight & says to dd .. You don't want to get fat. (I don't agree with mentioning stuff like that at such an impressionable age)!
I can't interfere can I? 
Dd has come home from their house & has cried about the gf & I hate that.. I've just said try to forget she's there & enjoy your time with dad