Rent relief query

I am living with my sister and her daughter. We are renting a 3 bed apartment from a rental agency.
Both of our names are on the lease. She works part time on a Fas scheme and gets her weekly lone parents allowance. She also receives a small maintainence cheque every week.
I am looking to claim rent relief for 2008. The rent is €850 a month and I pay €500 of that a month. I am in full time paye employment.
My question is can I make a claim for the full €850 a month as my sister doesn't claim for it. Or would she be entitled to claim for rent relief too as she is renting privately?

Why is this posted again? I already replied to the exact same question in this forum earlier on today.

You hardly expect to claim for 850 when you are only paying 500 of it?