AITA for setting a boundary and suddenly becoming the villain?

For context: My brother (29M), his wife (we’ll call her Cecil, 27F), and their 3 kids live 50 feet in front of me (36F) and my 3 kids. Both homes share one well (awful as you’d imagine). It’s just us – me, them, our mom, and our grandmother. That’s the “village”.

I’m a lifelong people pleaser, so I’ve rarely told them no. I’ve helped clean their cars, house, watch their kids (who are wild), and more. All I ask in return is to borrow our grandmother’s car occasionally or use her washing machine. I don’t ask them for anything else – and I’m raising 3 neurodivergent kids alone.

Cecil’s always been difficult – controlling, entitled, and constantly starting drama. She’s trashed two of our grandmother’s vehicles and now drives her new one. My grandmother has even said she feels pushed out of her own home.

My relationship with Cecil is hot and cold. Every other week, there’s a new problem, real or imagined, that she takes out on me. I usually let it go, but here’s what happened:

Last week, I was mid lunch with my kids when Cecil came demanding I go sit at my mother’s house for hvac service – bc she wanted to take her kids to the water park. I told her I couldn’t – I was cooking, didn’t have a vehicle, and wasn’t dragging my kids into a hot house.

A few days later, their daughter had surgery. While I had 4 kids (my 3 + my nephew) bouncing around my tiny house, Cecil messaged asking me to take her youngest, too. I told her no – I was already maxed out.

Now here’s where it blows up: I’d been asking for days to borrow our grandmother’s car to get food for my kids – ignored, then told no. So, baby daddy brought me a vehicle to use. While I was gone, Cecil came outside and yelled at him IN FRONT OF MY KIDS saying I’d never use the car again, never wash clothes there again, and once the well’s dealt with, my water will be shut off too.

This isn’t the first time she’s threatened me or used my kids as leverage. She’s told me and my mom that my dad’s passing was our fault, and that she “doesn’t blame him.” She’s insisted on helping with childcare, only to bail and call me home minutes into my shift. But I’m supposed to handle all the kids, no problem.

I almost reacted. But I’m done giving her my peace. If you’re mad, fine. But don’t drag my kids into it. And don’t run your mouth about adult issues in front of them.

She just got baptized last week, and I really thought things might change. But I’m beginning to think it's just how she is.

I know I’m not wrong for saying no. But I still feel bad. I know what it’s like to need help and not get it. But unlike me, she has friends, family, a husband. I just have them.

So why am I always expected to help, and then be punished when I can’t?

AITA?