If the power of attorney is revoked, the existing attorney's will remain in situ until the paperwork for the replacement attorney's has been authorised by OPG, it will be a seamless transition and no need to worry.
Any problems encountered, are more likely to be from the banks; the outgoing and incoming attorney's should work together to ensure mum has access to her money whilst the banks are updating their records.
It is of course, an attorney's duty to put mum's best interests foremost and it is tiresome of your sister to squabble about who would be a better attorney.
Your mum had capacity and it was her decision alone to choose who acts on her behalf, never mind what anybody else thinks and the OPG will stand firm behind her appointed attorney's.
A Lasting Power of Attorney is put in place to provide advice and support during future times when mum may have trouble making her own decisions.
Attorneys can be revoked, if mum still has full capacity and that is her choice, but be aware that the OPG do not accept a revocation lightly. They will more than likely seek independent medical reports that mum made the decision herself.
This happened with my mum, her original attorney was, shall we say, highly unsuitable. We had to jump through the OPG hoops, but the revocation went through and we got him off mum's back, not before he had helped himself to her pension money plus, plus, plus, but that's another story.
If your mum doesn't have capacity, then the process is a completely different kettle of fish!