No-one is obliged to be an executor. Your sister can sign a form removing herself as executor (called ‘renouncing’ her executorship) and file it at the Probate Registry. However, she must not have interfered in or taken any part in looking after the estate — what lawyers call ‘intermeddling’. Once she does this, she loses the right to renounce her executorship and must carry on.
A solicitor can draft a renunciation for your sister.
Another option is for your sister to say she wants ‘power reserved’ when applying for a grant of probate. This means she can legally leave it to you (and any other executors) to administer the estate, but may be called upon later if you and the other executors are unable to carry on for any reason. You should take legal advice before taking this route.