Originally Posted by JJeffrey

Generally yes with the exception of bags:

Seating: no problem for him to call AA and get your sister's party seated with him in MCE/preferred seats

Baggage: with 4 people in the other party I doubt the PLT allowance will be extended to his sister's group. This is completely up to the check-in agent. Sometimes with 1 or 2 companions a friendly check-in agent will not charge the others for bags, but with 4 I wouldn't plan on it.

Priority Security & Boarding: they will not share his Group #, but a simple "they're with me" works 99% of the time (even your mother on the same PNR will not inherit his Group #).

Award Fees: the only non-elite fee is for booking within 21 days, so no issue there, assuming you're booking this online.