There are devices that can help you transfer her. A sliding board serves as a bridge from wheelchair to car. You put one end of the board under your mother's
buttocks and the other side on the seat of the car, then slide your mother over to avoid having to lift most of her body weight. But its effectiveness depends on the make of your vehicle. A sliding board works fine with most cars, for example, but not with SUVs, which are too tall.
Most important is that you have someone show you the right body mechanics, which involve using your legs instead of your back as you turn to move your mother from wheelchair to car. I'd recommend that you consult a physical therapist, who should be able find the best method for you. Contact your local visiting nurse services, home health agency, or rehabilitation center, and ask them to put you in touch with someone who can train you to transfer your mother safely. When you're being trained, it's important that you use the vehicle you'll be using to transport your mother.