I agree with your approach and am sorry she is being hurtful.
Best way to deal with it, in my opinion, is either ignore her if she is beyond reasoning with (or if you think she gets some sort of kick out of putting you and your sister down...in which case I would seriously start to limit the time you talk to her/spend time with her) or, if you think this is some sort of temporary workout insanity, be upfront and tell her that whether or not she means for them to, her comments hurt you and it needs to stop if she wants to have a relationship with you going forward, period.
I'm lucky to have a super supportive mom. She needs to lose less than I do but we are in a, healthy, competitive kick with one another while still supporting each other's goals and accomplishments. I don't know why the people who "love" you are the most hurtful sometimes...been there done that with my grandmother (who I loved dearly but was not really nice when it came to talking about my weight or my sister's weight).