Thanks for all the replies.
Just to clear a few points up. The money is stolen, If this money is used to pay companies, i.e. a bank over draft can my mum then claim that money back from the bank, like you could if i found the person with my stolen bike?
From my point of view, i don't care that my sister has stolen the money and now has 15k from our in heritence, what i actually care about is that my mum sold her larger house and downsized so she had money for her retirement, she is now 61, she i salso ill and goes on holiday every year and takes her career, which costs about 6k. Myu mother is now left with about £1000 cash and that is it. She had plans to install a new disabled friendly bathroom, which now have to be scrapped.
In a few years when I have finished my second degree and climb the ladder a bit, i'll be in a posistion give my mother money.
My mum feels so insecure now that she has no finacial back up.