OK, how long will her mother be wanting be with her?
As you certainly can´t have them both!!
Is she the youngest by any chance?
Just thinking a little about the selfish thing.
I think a lot of us want things our own way-does that make us selfish?
Given the chance I can be selfish and have been known to take advantage of people who let me.
So it can sometimes be the way a person is treated.
Let them get their own way enough and they expect it.
Sorry, OT.
So be kind, helpful, tell her you will do what you can etc, etc....
Does she get on with her Mum?
Perhaps she wanted to stay with you so that she´s not on her own & a sitting target for Mum to stay/visit when she wants!!
When does she arrive?
Perhaps you need to tell her before she arrives that she can´t stay?