I call BS. If he was "insecure" about his back he wouldn't walk around frequently with his shirt off. Plus being a bully in making jokes at the expense of your appearance and following with "oh, take a joke" means he better be ready to eat what he has been dishing out. Should've thrown a joke about mom in there too since she also likes to join in on the quote unquote "jokes". But you being a more mature adult than I at 28 am, even took the highest of all roads and apologized, your definitely not the jerk, and if I was you, I'd lay into them just as much as they do you. They crack "a joke" about you, force a giggle then force one back AT BOTH of them.
She obviously wasn't hard pressed for a sitter during that week. She does owe you money and you weren't the jerk she is crazy. Also to the comment about not doing chores while watching the kids, she said the one kid was the only one there at that point, maybe got dropped off early, in the normal time for mom to make her bed, besides it's a home daycare, normal house stuff is going to happen
Oh wow even my SIL had more sense than that. She wanted my husband and my daughter in her wedding and invited me to be in it as well because even she (toxic af) knew that wasn't cool.
But your sister is giving off MAJOR creeper vibes dude