What about (I'm actually serious about this)...

Go to Home Depot and getting a pulley system. Ask your brother to take the stick out of his butt and install it on the ceiling into a stud. At one of the loose ends of the pulley, tie a towel or something that your mom can slip her legs into, and then whoever is assisting her can pulley her legs up enough to slide over the edge of the tub.

Actually, it might not have to be an actual pulley. A large eye hook might suffice as well. It's not like her legs weigh 200 pounds or anything -- you just need something to give you leverage.