This is indeed as hard situation and something I have been through before also. But here is some things I think you should do.
1. Talk to her and find out
why she hates you playing the piano (I have a feeling that she tried playing the piano before and really wanted to be good but never got it so she is taking it out on you but I could be wrong about that).
2. Find out when she is home and when she is out of the house, you practice when she is not around (not the best option to be sneaking around your love in the first place)
3. Many people think that pianists are just very soft and that they can just push them around. That is kind of true but we need to prove them wrong. You can stand up and put it right out there... "honey, I love you but I also love the piano. If you love me then you need to also respect the things I love too. You don't have to love it also, you just have to respect that I love it." Something along those lines and see how she reacts to it.
It's never healthy trying to have a relationship with someone who is constantly trying to take away those things you love to do. If she is not giving in to your demands of playing piano then you will have to make a decision. "Can I live with this woman and not play piano or does playing piano mean much more to me then pleasing a woman". I promise you will find someone better who will just love to hear you play and will constantly ask you to play for her.
Hope this helps
