Absent any prohibition in the Power of Attorney instrument, a person who has been convicted of a felony may be named as an agent (or attorney-in-fact) by the Principal (the person creating the Power of Attorney) in Maryland. However, the instrument could be written to disqualify an agent if they are convicted of a felony if the Principal has a concern. If you are choosing to name someone who has been convicted of a felony and if there is concern among other family members, a solution may be to require that the agent provide regular accountings regarding how he/she is managing the assets/affairs of the Principal.
You should contact an attorney licensed to practice law the the applicable state to advise you specifically regarding your set of circumstances. While there are a number of research resources available, you would likely need to check each state's legal resources - such as the state code - for information this specific. I am not aware of a single resource that would address this issue in each jurisdiction.
Best of luck.