tax return: own a home, but splitting a mortgage...??

[nashgirl21]

I bought a duplex with my sister 14 months ago. We are "tenants in common", which means we co-own the home. However, the mortgage is entirely in her name... I just give her half of the mortgage at the end of every month and she pays it in full.

My question is: how can I get the tax break for forking over the mortgage payment(s) when the mortgage is in her name? I know it's a pretty big difference that I don't want to miss out on. Also, what would I put down for my mortgage payment? Can I write-in what I pay, or do I have to put "zero" since I am not legally bound to the mortgage (even though the mortgage is for a home I co-own)...??

I'm SO confused!