it's a toughie.i would keep encouraging her to play with other 'kinder' children,but tbh,all kids-even lovely one's-have the potential to be bratty or mean on occasion.
my dd is 4,and is very kind,polite,sociable etc but is sometimes influenced by other children at nursery.
like your dd,mine is picked on by 2 girls at nursery.."your shoes are ugly/hair is horrible etc...yet she still wants to play with them
i told her she should play with her other friends as these girls didn't sound very nice..
but nope,she persisted.
i spoke to nursery about it and they said they all seemed to play nicely together.however,i have noticed mean spiritedness creeping in,in the way she plays with her 2 year old ds..i don't know if it's just a phase that all kids go through,or if she's mimicking these girls..
i think all you can do is encourage them to stand up for themselves.they have to learn about self preservation at some point...
but no.i would refuse to invite them round to your house..like i refused to have them at dd's recent birthday party