I am a solicitor in Scotland. Unfortunately, the agreement with your mother and stepmother wasn't properly set up initially in my opinion. A will is all very well but as you have discovered it doesn't work if someone changes it. What should have happened was that you should have taken a security (charge) over the property and a loan agreement should have been made in writing and registered in the Books of Council and Session for preservation and execution. None of that was done and the loan you made was more than 5 years ago so the obligations there, presumably verbal, are time barred. The only saving grace here is that the property went to your children. You have no case for making a claim I'm afraid. Happy to discuss further. Please leave a positive rating so that I am credited for my time.