I, too, understand. My mom's undergoing chemo, and the burden falls on my dad. I've taken some of the responsibility, but feel like I could do more even when my parents claim there's nothing they need. On the flip side, I'm rejecting my friends' offers of aid because I can't come up with anything to ask that doesn't seem like an imposition.
That said: Spinr and Marie55 had great suggestions. I've a couple more: stop by and fix dinner one night. Take your mom out to dinner. Come by sometime and water her houseplants. Call to say you love her. Despite all the offers of aid ("If you need anything, just ask!"), I've had, only one person has really done anything beyond that. When I was overwhelmed with stuff at my folks' and at work, she repotted two of my plants at work, both of which needed it, both of which I'd been neglecting. I appreciated it more than I could say.