Ugh!!

Every time my step kids are here my husband and I are fighting. In between every thing is perfect. When they’re here he’s his ex’s errand boy. And sometimes inbetween. Ex: she doesn’t want them to be at camp all day can he pick them up for her (but our baby can sit at daycare for 8 or 9 hours) or she “forgot” to bring their backpacks for camp so he’s driving 30 min to pick up the back packs. Twice in the last week this has happened. I feel like I’m married to someone else’s husband. Its so unattractive how much he bends to her every word. Is it me??? Idk what to do anymore. He gets upset wth me and says his kids shouldn’t have to go without because she’s incompetent but where does it end?? (Also they would not go without theres backpacks and lunch boxes here, just not the ones they usually use).