possession and poltergeist activity are actually rumored to be the affect of pent-up emotions (generally anger and sexual fustration), but in a child of 8 or whatever, i can't see why that would apply.
if it has something to do with the house itself, then the child must have a specific link with the "being" that is communicating with her (by specific i mean age or something).
i had the same sort of thing for a while, it's not so bad now. I had a lot of pent-up anger, so my "spirit" friend was just a manifestation of that anger. It used to rip things off the walls, break glasses and stuff, and it pushed my little brother down the hall. It used to "love" me though. If I was sad, it would sit beside me. It was like an invisible sibling or something.
If you believe in psychic phenomena, then the case your friends sister could just be mental powers and/or emotions just releasing itself. And because of the age, and that it's been happening for a while, it might be coupled with an overactive imagination that can't express itself in an artform of some sort yet.
I don't recommend the priest though... it doesn't sound serious enough for that. Just try to encourage some sort of artistic expression. If it gets worse, maybe then call a priest. Or, i've been told there are some psychic investigators around, you could try that...
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