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The starting point is that in English law, money in joint accounts automatically passes to the joint account holder on the death of one of the joint account holders by survivorship. The position has been clarified by the decision handed down in Re v the Estate of Edith Mary Northall Deceased in June 2010 by Mr Justice Richards who decided that that, where a sum of money belonging to a mother was paid into a joint bank account with her son (in this case), there was a presumption that the mother did not intend to make a gift of it to the son on her death. Accordingly the money was held on resulting trust by the son for his mother.
The joint account was opened for reasons of convenience and no thought or consideration was given to a right of survivorship by the son, ie the automatic passing of the account balance to the him on his mother's death. Although the account opening form provided for survivorship (ie the account would pass to the survivor upon the first party dying), this did not assist the son's argument, given that there was no evidence that this part of the form was explained to Mrs Northall or drawn to her attention. There was no agreement between mother and son to the effect that the money was to become his as the surviving bank account holder. The balance of the account at the date of Mrs Northall's death therefore formed part of her estate. Her son was accountable to her estate for all the amounts paid out of the joint account in his favour, except for sums which, on the evidence, were found to be authorised by Mrs Northall herself.
In your case, the facts appear similar and therefore I believe you have a good case to chase your sister for the monies. Hoever, as she has already closed the account, it is whether she will willingly hand over the monies. Litigation can of course be very expensive and may not be worthwhile for what you are trying to re-coup.
I hope this sets out the legal position to you. You would be advised to see a local Solicitor if your sister is not willing to part with the monies.