family issues

I have a brother who has been with a gf for 7 years. After they left Uni they came to live with me, in a house my dad owns and when I discovered I was pregnant they wanted to move out. They wanted to get a mortgage together and were given a hefty deposit by the gfs mum (as her grandad had recently passed away and left a massive amount of money) But at the time my brother had 2 part time jobs, so couldn't get a mortgage, so I agreed to help my brother out, as I was full time employed and could go on it with his gf. It meant I was on the deeds but had no financial gain, my brother paid, I was just on it for the paperwork to help him out. Fast forward 2 years and my brother finally had a full timer which meant he can now swap with me to go on his mortgage.. But his gf has now refused to change to terms of the mortgage, which means she wants my brother to be on the deeds, but not have any percentage financially Which means she has100% of the house but my brother would be 50% liable to pay for the mortgage. He is not happy, but she's basically telling him, if he doesn't sign it she'll kick him out and it'll be the end of their relationship. I think this is emotional abuse towards my brother. Who in their right mind would sign up to pay a mortgage with those terms! She won't even agree to put in a disclaimer that if it's sold, she recieves her deposit that her mum put in, but the rest is split 50/50, even though they would both be paying it. She has called my brother a 'glorified tenant' and made him feel worthless. I have refused to sign the mortgage over with those terms as literally my brother would be doing himself over! Yet.. My mum and dad don't say anything.. They treat my partner horribly, he's a fantastic father and has never done wrong by me, but my brothers gf is treated with respect, even though they know what she's like. I would have NEVER agreed to going on the mortgage if I thought she would of done this and this is not what I agreed too. I did it as a favor to my brother, not her! Yet she would benefit from it if anything were to happen. This is causing my to loose sleep, cry continously and I feel Im being treated very differently to my brother. My parents are always quick to jump down my throat and say nasty things about my partner but will not even say a word to my brother. I literally fear for my brothers mental health and fear he is being taken advantage of. He has been upset for weeks over this and doesn't feel like she's being reasonable. My parents are fully aware but haven't spoken to him or anything. When I brought it up to my dad, he said, don't sign the paperwork.. Which means I will be stuck on the mortgage with his partner. My brother is being cohersed into signing it. When I spoke to him he agreed that he didn't agree with it and said if it was the other way around and it was me, he wouldn't sign it. They are trying to go with a different mortgage company, but it's been held up, so they are paying alot more as they are out of contract. I don't know what to do!!! Do I sign it over to my brother and watch him be taken advantage out of or refuse to and possibly get myself into trouble. I don't know my rights, I have no idea what will happen if I don't agree to sign it over. Help!!!!