My sister has two kids and asks me to baby sit sometimes. The thing is the favour is never returned. Now before people say do I ask her well, it isn't actually a question of asking as every weekend she goes clubbing then likes to "rest on a sunday" (her words) and every school holiday she's off on holiday (is at the moment and it's her second holiday of the year) so there is never an opportunity. Now a few days ago I picked up some food that was part of a deal that me and my kids don't eat but came with what I bought anyway. So I messaged her and said "got some burgers for the kids as part of a deal I will stick them in the freezer and you can pick them up when your able to." She messaged back "oh good they can have them on Saturday when they come to yours" I was like 
it's the half term, I don't get a break from my kids ever as their dad is absent, hers goes to their dad she gets lots of breaks. I don't begrudge her but she didn't even ask. I read it and didn't respond as I was annoyed and actually trying to think of what to say back to it, before I knew it she called and was speaking about something else then said "when they come round to yours on Saturday" I was just like okkkk by that point it felt to akward to say anything. Aibu to tell her that I won't be having them again? Or should I just say that she should help out more with mine? I know either way she will be offended.