sister is more valuable than the money

I can understand your feeling. It would be very hard to save this money. With this money, you can do a lot of things for you and your kid. Without this money, your life and your kid's life will be much harder. So tell your sister you need this money more than she does. Law suit will not solve the problem. Even if you wins, how can you collect the money? Maybe, your sister does not have any money left.