Generosity with strings attached is the worst.
As Normal Rockwell it may appear to them to have their adult children living together as one big happy family, it does seem a bit farfetched.
I know what you mean about not wanting to appear ungrateful, at the same time they tossed in a quasi stipulation. It comes with conditions.
Are you able to adjust in other ways? Take on more work hours to help yourself with bills? Take on a roommate? Go find a place looking for a roommate?

Doesn't sound like you need the help that they are offering.