Can the same lawyer who did the POA paperwork for a client also sign as a witness to the POA form?

That should not be a problem, provided the attorney is not receiving any powers from the POA or otherwise a party to the POA. If you have concerns about your mother's mental capacity, I would advise that you retain an attorney as making such a claim will be virtually impossible without legal representation. Moreover, it will be difficult to prove without some form of medical evidence. Her physical condition is not likely to be relevant to the determination of capacity.

It is common for an attorney to be a witness or notary on documents. It is also very difficult to question the judgement of the mental capacity of a person sometimes. Many clients can have good days and appear to be able to understand what they are signing and POA's can only legally use the document to assist the one that granted the powers.

I agree with both of the prior responses. This is very commonly done. I would add that the lawyer is generally very well-positioned to determine the legal capacity of the client, (who is your mother, not your sister), and to make sure that your mother is not subject to any undue influence from your sister. James Frederick

Most attorneys who practice in this area of the law understand very well that the actions of an older person who is frail may come under scrutiny after the fact from relatives who did not approve of the the will or the codicil or the POA or the trust, and that they and/or their support staff may have to testify in Court, and therefore they take particular care to make sure that the signer understands their actions and the effects of their executing the document.