How to say no to babysitting without causing conflict

My husband’s sister frequently asks us to babysit her two kids. She’s a single mom, so I understand she doesn’t have many options. But her kids are... intense. Without going into too much detail, their behavior feels downright disrespectful, and I often feel bullied when they’re here. They’re old enough to know better, which makes it even harder to deal with.

I already have two young kids of my own, and I simply don’t have the energy to handle her children on top of mine. My husband agrees with me, but he’s very conflict-avoidant, so I’m always the one who has to say no when she asks. She asks a lot because she doesn’t have anyone else willing to help. Honestly, I get why—it’s exhausting.

She asked again today if we could babysit next weekend, and I said no. Then she pressed me, asking, “Why do you always say no?” She added that it’s been a long time since her kids were at our place and that they “miss us.”

How do I respond to this without sounding rude? I want to set boundaries, but I also don’t want to escalate the situation. Any advice?