Why does my stepson’s mom get to skip her parenting time, and I’m just expected to deal with it?

I’m a stepmom, and I’m seriously burned out. My stepson is supposed to go to his mom’s every Tuesday and Thursday from 3 to 8 PM court ordered. But lately, he keeps saying he doesn’t want to go… and she just goes, “Okay.” No pushback. Just “cool, I’ll skip my time again.”The other day, he said he wanted to stay home to play Fortnite and she AGREED!!!! Who does that?! What kind of mother hears that and says, “Sure, I won’t see my son today so you can play a video game”? And of course, when she doesn’t take him, guess who ends up taking care of him? Me! I’m with him all the time. I love the kid, but I need space too. I can’t even relax in my own home because I’m always in “mom mode,” while his biological mother gets to peace out, guilt-free, like parenting is optional when she feels like it. And now it’s becoming a pattern. She’s not co-parenting. She’s barely parenting at all. I’m at the point where I’m ready to start setting serious boundaries, because it’s not fair for her to keep skipping out and dumping everything on me and my fiancé. Anyone else dealt with this? Tell ME!