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Provided your sister left over £5,000 in assets, the Executors named in the Will are under a duty to obtain Probate, which is evidence to the Bnks etc that they are legally entitled to collect in the assets.
If you do not wish to act as Executor, that is fine, and what you will need to do is to "renounce Probate" which in simple terms means that you want to give up your position as Executor and allow your sister's daughter to do everything herself. There is a simple Form for you to sign renouncing Probate, which your sister's Solicitor can prepare.
Once you have signed the Form, your sister's daughter will then be free to deal with the Estate as the only Executor.
I hope this assists and set sout the legal position.
However, please let me know if you require any further clarification.