1. Clearly the police ought to be interested and do something, but that would take some time and meanwhile you would still have the problem of having to pay the loan when you shouldn't have to. However, it would probably be a good idea to make another attempt to get them to become involved, & if they still won't to make a formal complaint to the Chief Constable or IPCC. But I would not leave it just to them.
2. It seems you need some competent legal advice, and haven't yet had any - apart from Barmaid's comments. You could ask the Law Society to recommend solicitors who are experienced and competent in this area of law. Alternatively, do you or your mother have legal expenses cover with your house buildings or contents policy? If so, it might cover this type of thing & you should raise it with your insurer. You then wouldn't have to pay for the legal work.
3. If you do have to pay, you could perhaps raise the money by stopping paying the loan. Ultimately, if you win the case - or the lender backs down when the reality of the situation dawns on them - you should get your legal costs back - & maybe everything that has been paid on the loan.
4. Do you know who the people are who witnessed the fraudulent signature? If so, are they prepared to make a statement confirming what actually happened? This could be vital evidence & as much info as possible about it should be given to the solicitor.
5. I don't understand why you & your sister signed the document. If the house does not have your names on the deeds then this seems to have been unnecessary, unless you were living in the house at the time (when you might have been asked to sign to prevent you objecting if repossession became necessary). But you say you have now moved back in, which seems to imply you weren't living there.