If they haven't contacted you about it, perhaps the sister found it odd that someone would volunteer another person for something like that. I wouldn't contact them at all. You didn't talk to them, your mother did. The only response could be to your mother..."Mom, I'm sorry you mentioned that to them. I just can't do it. Please let them know and you might suggest some other options for them, such as you providing some meals, Meals on Wheels or their church family helping out."
I agree--lots of people in the sister's position would not pursue that--I know I wouldn't! I'm wise to the whole "parents volunteering adult children" thing--mine don't do it to me, my ILs don't do it to us, and if someone else does, I simply ignore it.
Or, if I truly did need the help, I would approach the volunteer-ee and say, "Your mom suggested that you might be able to help us. Is that possibly true?"
And I think the reason your mom should call the sisters is that it will help her learn to not do this again. Sort of like making the teenager come back to pick up her own jacket off the living room floor.
(If you did want to help them, get them some takeout coupons, if you can. Or suggest a Meals on Wheels-type program, or remind them to check with their doctor. Or contact Elder Services or some such and asking them to pursue it. I'm feeling so terribly bad for the poor 90yo sister!)
If they did contact you, then you should simply say, "I'm sorry--my mother spoke too soon. Right now is a tremendously bad time for me to take on a responsibility like that. I can't have other people counting on me like that right now. However, have you looked into Meals on Wheels, or discussed this problem with your doctor, or someone? There *are* resources available, and some of them are ones we all pay taxes for. Or, people donate to organizations like that specifically because they want to help."