I’m sorry, this is an awful situation to be in. They are not being kind or fair or family like in any way at all.
But, kindly and gently, this is a little of your own making. You are doing as they want, every single time. For YEARS. I’m guessing from your answers, you have never raised it with them that you don’t like doing it.
So from their point of view, you are absolutely fine with the status quo. Why do they think that? Because they are CF’s, so will have very little insight into how the other party (you) feels. And you’ve never said a word about not wanting to do it. Therefore, they can easily convince themselves that they are doing nothing wrong.
You have a choice. You have always had a choice. So exercise that choice.
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you continue to keep them happy at your own expense, and so must stop complaining about it and continue to suck it up. Just remember that you are CHOOSING that option - with all its consequences and downsides.
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you decide that 10 years is enough, pull up your big girl pants and put a stop to it. If you are only in your twenties, it is getting on for HALF YOUR LIFE spent serving someone else with no benefit to you at all. Get angry.
They are of course going to be angry at you. Anger will be their weapon of choice to control you, guilt you, and get you back in your box. It is bullying plain and simple.
Do not give in. Not this time, and not ever again. I’d send a text message first being incredibly clear. Do not apologise. Be factual. Let them then come to you once you have sent it; no follow up messages from you at all.
‘hi xxx, lovely to see the kids last night, hope you had a nice evening. After much thought, I’ve decided that I’m taking a step back from babysitting for you so often; much as I love you all, this has taken up a lot of my time over the last ten years and it’s time for me to prioritise my own life and future; I have Big Plans! I’m sure you understand, look forward to seeing you at (THE NEXT FUTURE EVENT WHERE YOU WILL SEE THEM NOT AS A BABYSITTER)'
Best of luck, this has to end. Stay strong.