What about if I just get one independant witness and one of my sons? Also if I do become POA will she be able to still run her own finances as well as me having access to them? I am concerned that there maybe times when she is unable to physically get access to her money
I would make an appointment with a solicitor & discuss Powers of Attorney with them, or make contact with your local Citizens Advice Bureau to discuss the in & outs of a power of attorney. I'm sure one can be written so it only comes into effect when your mother is incapacitated or ill.
If your mother has no objection to one of your sons witnessing the signing of the document, fine but ideally as we've all pointed out neutral witnesses are best.