When you arrive at the restaurant you can ask that you be seated together. There is no guarantee, but CMs at the podium will do their best. It will depend on what tables are available at the time of your ADR. 'Ohana does have tables for 3, which might be what your 2 groups are assigned, so it will depend on the availability of a 6 top or 2 adjacent smaller tables. Be prepared to wait longer since 'Ohana is almost always fully booked.
There is no point in calling the dining line to make this request because they cannot combine 2 separate ADRs. The best they can do (and you can do the same thing online) is check to see if a reservation for 6 is available. If it is available you can book that reservation (either on the phone or online), cancel your res for 3 and have your Mother also cancel her res for 3.
