@Sunnyside226 oof, sounds like your brother is unable to respect your boundaries! Good on you for saying no, not always easy to do but we gotta do it sometimes hey! And considering how stressful it can be to have to look after little ones, it sounds like you've made the right choice. He can be pissed off all he likes, but it's not your responsibility to always be just, available and ready to jump into babysitting whenever he wants!
And with your sister, sounds like a pretty heavy situation. I hope you're okay.
Do you have anything on tonight? Could you maybe go for a walk or something else, whatever you feel might help you expel some of this restless, shaky energy?