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It’s very difficult to answer without knowing the full background.
In my opinion you are making a statement not an offer to pay.
You said that you’ve come up with a figure including interest to be restored. You are not saying that you will agree to do this.
For there to be a legally enforceable agreement, there needs to be an offer and an unequivocal and unqualified acceptance. Therefore, even if this does constitute an offer to repay, it appears that no one has yet accepted it and an offer can always be withdrawn at any time until it’s been accepted. You cannot unilaterally make an agreement. It needs 2 parties.
If the siblings had said to you, okay, put that money back into the account, I think you may struggle a little.
I don’t know the circumstances of this money being removed from your mother’s account and why it needs to be repaid.
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