Always feel bad for saying no

So my father in law asked us to babysit his daughter (my husbands little half sister) this week while they go on a trip. Again. We’re always there to say yes even if that means canceling plans and making accommodations. They’re two towns over and we always have to go there due to her being back at school. We just got back from a week long trip across country and next week will by my husbands last week of his vacation. I want him to enjoy and relax while he can before he goes back. And to have time for us and himself before baby comes in a few months.

I have a hard time justifying a no in my head. I dont know how to say no without it sounding bad. So I always try to justify it but if I can’t; I say yes. This month we have a lot to do before the baby shower and getting things ready. But I know he’ll use the it’s only a few days excuse and yes they do pay us for watching and taking her to school ( we’ve repeatedly told them it’s not necessary) i dont know what to do. I’m in the last stretch of pregnancy and I’ve been having a lot of pain in my back so I honestly don’t feel up for it. But i dont know if we should just say yes or if we should be able to say no and not feel bad. But I dont know how to say no and not feel bad.

UPDATE: We said no. Of course he tried to make us feel bad by saying we could’ve told them sooner. And threw a little fit but we said no and stuck to it. Thanks everyone for the courage!