I think it's crummy the parents didn't talk to you about this directly, but I'm inclined to think that part of the reason they hired a babysitter in the first place was so she can be the one to enforce homework (and then they won't have to). I've seen things like this many times in my babysitting career. It stinks because not only does it mean twice as much homework on Tuesdays and Thursdays, but it also means more of a struggle to get her to do it in the first place (it's not part of her daily routine; "mommy and daddy don't make me do it, so why is my babysitter being so mean about it," which then rubs off on how she sees you, etc).
I agree that you should discuss with MB and DB. But be prepared for them to ne dismissive about it, and either make excuses/defending themselves or to just straight up tell you that they're expecting you to do it so they don't have to. I think it's also possible they will apologize and tell you they'll try to enforce homework more often, but again, be prepared for the fact that that might not actually happen.
I don't want to use the words "suck it up," but...I think it's just part of the territory of being a part time babysitter. You can quit and work with a different family where this might not be an issue, but really, having to help her with homework (even if she fights it and you have to do twice as much) isn't the worst thing you could be dealing with here.
Lastly, it sounds like you are also with her on Fridays when she has no homework. Perhaps you could make an agreement with her that if she either does her homework on Mondays and wednesdays when you're not there, or at least doesn't put up a fuss about doing it when you're there, then on Friday you guys could do something extra special (bake cookies, watch her favorite tv show, get ice cream cones, etc)?