What can i do about my mother not exercising?

Encourage her: Some people do not exercise out of habit, being busy, concerns that it's too challenging to start, pain or stiffness (as in arthritis), or just personal choice. It's a good idea to encourage someone to start doing something that he/she enjoys (eg dancing, bicycling, walking with a group of friends). It might help to give her a gift card for some classes, but make sure her doctor approves.

Other ideas: Maybe just get her out walking with you or having some of her friends invite her as a social activity. The best idea is to find something fun that she may not consider "exercise" but still reap the benefits.