There seem to be a few issues here.
(1) If your uncle and aunt owned the house as "tenants in common", then he can give his 50% away as it's completely up to him. BUT, if they owned it as "joint tenants", then the instant he died, she owns 100% and it doesn't matter what the will says (because, in a joint tenancy, the house goes to the survivor and doesn't form part of the estate that passes under the will).
(2) If your aunt has signed something giving away her share of the house, then the effect of that will very much depend on what she signed. If she gave it and didn't receive anything in return, then it is quite possible that this would not be legally binding. You need to get her to talk to a solicitor. If she can't afford one, your local citizens advice bureau should be able to tell you where your nearest free legal advice clinic is - they might be able to help. Directory enquiries shold be able to help with a telephone number for the citizens advice bureau.
Even if your aunt doesn't own the whole house, she might still have a right to occupy it, under the family law act. A solicitor could tell her more about this.
Good luck!