Maybe don't bring it up around your stepfather....plus, there's less chance of embarassing her if it's just the 2 of you. Maybe you and your mom can go out, and spend some quality time together, and you can approach it from a concerned angle, so she doesn't feel attacked or like she's being criticized. And I think offering to exercise with her is a good idea....it will keep her motivated, because your motivation is keeping her healthy, and her motivation will be you pushing her.