It sounds like your plate is full and anyone should be able to see that. Hopefully if you explain it to your older daughter, she will understand. I would not want my parents to take on more than they could handle (and become unhappy/resentful because of it). However, I would appreciate if they told me their reasons. "No" is all you *need* to say, but explaining could help smooth the relationship.
Here's an example from my own situation. My mother retired shortly before we adopted our first child. I was worried about child care because we didn't know how quickly we would need it (didn't know when a child would be placed with us) and everyplace had waiting lists. I got on the waiting lists and was hoping for the best, but my mother kept calling to ask about my child care plans. I told her I was on waiting lists. Every time, she'd "remind me" that she was NOT available for childcare. She was so adamant about it was weird, especially when I told had told her that I understood her position and promised that I would not ask her to babysit. (I hadn't planned to in the first place.) I can understand why she wouldn't want to spend her retirement babysitting, and that's fine. (As it turned out, the adoption took a while and we were able to get into one of the centers from the wait list
and even if we couldn't, I'm sure we would have come up with something. I had never considered asking my mother to babysit.)
Then a year later, my sister had her first baby and my mother took care of my niece full time when my sister went back to work. Something must have changed, but I don't know what it is (my sister was fully able to afford child care, she lives about the same distance from my parents, etc.) My mother is fully in her rights to say "no" to me (even though I'd never asked) and say "yes" to my sister. I could never figure out a way to ask "why?" without sounding petty, but I have wondered. It would have been nice if she would have explained her decision -- especially since she was so adamant about it -- even though she doesn't "owe" me an explanation.
My parents and I have a good relationship -- they help me out when they can, and I help them out when they can (a good "two way street"). This situation has not ruined our relationship and I try not to think about it often (especially since it was years ago now), but when I do think about it, I still feel confused about what they did since they did not explain their decision.