What level are you at in Crown and Anchor? If you are Platinum or over you, and your mother, should get priority boarding. However, there are so many Platinum and higher cruisers it may not be worth it. On our last Royal Caribbean cruise on Freedom OTS the lines for regular check-in were shorter than the "priority" lines.
If you are eligible for priority boarding I would just have your mom go along with you. If they don't let her check-in with you I would explain you are in the same cabin and she is your mother. If that doesn't work then just go to the regular line.