Oh, the mom's who hate saying no. My mom was one of them, a complete people pleaser, especially to her daughters.
We had to finally get to the point of saying. "Mom, seriously, it is fine. If you keep going on like this when you have to say no, I'll quite asking you for help. Now be quiet about it."
Then you call a few days later and say, "you know it was a good thing that you couldnt sit for us because I found this great holiday camp at the museum that the kids are really excited about going to and I wouldn't have looked into it if you'd stayed with them." or "after thinking about it, we decided we do want the extra time with the girls, so I'm taking 2 days off and DH is staying home 3 days. We are really looking forward to the quiet time."
If you dont do a follow up she'l not enjoy her time away because she'll feel guilty.