Sorry to hear of your problem.
On what basis are they claiming that you should return the money?
Is there any documentary evidence at all which speaks to whether the money was intended as a gift or a loan?
What was the value of this sum and how was it transferred?
The status of the money is the most important issue. If it was a gift to you from your father then you do not have to return it.If it was a loan then you will have to repay it on the terms you agreed.
If your father requested that half the money be paid to your sister then you will have to do so. If no such direction was made then you don't.
From what you have stated it appears that he gifted it to you. This being the case and along the fact that there is no evidence to the contrary means that it would be unwise and imprudent for your mother to issue a claim in respect of the monies if you decided to keep them. If the fact that it was a gift was can be corroborated by your sister then you should keep he "onside" as it were since she will help if your mother does issue a claim.
My advice would be to write back to them refusing to give the monies explaining that it was a simple gift from father to daughter and understood as such between both parties and, if appropriate, that it was to be kept by you rather than shared with your sister.
The worst case scenario is that they issue a claim to recover the monies and you will both have to produce evidence to the Court in respect of your arguments and it will depend on the weight of credibility the Judge attachs to either.
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