+1 for the „you don‘t have to do it“-camp.

In general, that is. Unfortunately for this particular weekend, you‘ve told your sister „If you absolutely can‘t find anyone else, I‘ll do it.“

I can tell you right now from my experience at work with swapping shifts: If you make that kind of statement, it‘ll be interpreted as „Yes, I‘ll do it.“ So take that as an advice for future conversations with your sister and also for your job at the hospital.

I think this time you should probably do it, because your sister really doesn’t seem to have another option right now.

Also, if there‘s an emergency, of course I think you should help.

Apart from that, though, I think your reasons for not wanting to do it are absolutely valid. I don‘t see it as you looking for „any excuse“ to get out of it, and I especially don‘t see it as a lack of empathy! Don‘t let some of these comments get to you. You have already helped with babysitting many times before, so it‘s not like you never do your sister a favour.

I completely understand working hard and looking forward to a weekend of camping, instead of quietly watching TV by yourself, hoping that the kid won‘t wake up.
Also, you having to give your dog for the night is kind of a big deal! It’s a hassle, and also it‘s YOUR dog who happens to live in your house, and it‘s stressful for the dog and annoying to you that you have to remove him/her.

In the future, it‘s probably best if you say right away, „no, sorry, I‘ve got other plans.“ Any attempt to justify or discuss it will make it easier for her to pressure you. I know, easier said than done – I‘m also still working on saying „no“.

I don’t think you are in any way being selfish in not wanting to change your own plans. You have a right to have plans and a right to not be forced to change them because someone guilts you. It isn’t selfish to have a life. I’m saying that as a parent of two. I never pressured anyone to watch my kids. I can’t imagine pressuring someone who is reluctant to watch either of my kids. It can’t be a good situation. It might be time for your sister to come up with more depth for babysitting, especially for overnight.

I also don’t think it is selfish or rude to expect to be compensated for your expenses. I grew up with the idea that if someone did you a favor it shouldn’t cost them money. Where I live it is standard to compensate someone for any expense incurred for a favor. It isn’t selfish or rude. It shows someone that you understand what they are doing for you and that it costs them and that you want to make sure that it costs nothing more than their time. This isn’t you making a profit off of your sister. This is you being compensated for the expense you would incur for the care of your dog while providing free care for her son. Also, if you have a babysitter around here you provide the meals. Even at daycare you send meals along with your child and you are paying the daycare to watch your child. You doing a favor for your sister shouldn’t include you spending your own money to compensate someone else for doing something like caring for your dog that you aren’t doing because of the favor. The same for losing money on camping. If you’ve paid for camping and are losing that money then she should compensate your for that. It is only fair. It isn’t greedy. You don’t ask someone to lose a deposit because you need them to help you. You ask how much it cost and give them the money without them needing to ask. That is what people do when they are appreciative of the help and when they understand that a sacrifice is being made for them. You giving up your plans is you making a sacrifice. You are the one being inconvenienced and you are the one who is being forced to give up their plans. It shouldn’t also cost you money. You shouldn’t be paying out of your pocket to do her a favor. She isn’t working for free. She expects to be compensated.