Let your tita get a taste of her own medicine without being rude. *wink* Funny thing is that in reality, they are not so fit as well. So jokingly, you can throw back the comment to her and share a good laugh together.
Careful though, some relatives are highly-sensitive to criticisms. Okay lang pag sila nagsasabi, pero kapag ikaw na, nako. If she is in a good mood, then you can probably say it, but if no, then just skip this response and try the others.