I’m so sorry your family is going through this very difficult time. I don’t think your mother is becoming depressed. I think she’s already there. She’s given up on getting better and doesn’t see the point of going through the pain of exercises or of denying herself the food she prefers to eat. You are very correct to be worried.
I think the time for tact is over. It’s time to work with her doctors and to let her know that she is letting you and your sister down. Stress that you want her around for your successes, your weddings and babies and to give you motherly advice. Let her know that you are willing to help her in any way you can — from helping her exercise to making the foods she should eat. Ask her doctor to arrange to have her meet with a therapist to help her cope. See if you can arrange for sessions with a physical therapist several times a week. Don’t take no for an answer. Sometimes loving someone means caring more about what is happening to them than they do. It’s a lot to ask of an 18 year old, but apparently you are the one who is there. If your dad is in the picture, by all means enlist his help! If you have other relatives around, ask them if they can rotate visiting to put your mom through her exercises and to have a healthy meal with her. You shouldn’t have to go this alone.
Some fights are worth having. Your mom isn’t going to welcome the tough love. But her life depends on it.
Meanwhile, it might be helpful for you to see a therapist, not because you are showing signs of mental illness, but because you need some extra support and perhaps some advice along the way.
I wish you well.