Step kids acting out only at moms house??

I have 2 step kids that are 12 and 13 1/2. My husband has custody so they live with us full time. They chose to move in with us last year, mom is craaaazy but not unfit or anything. They visit her every other weekend and then 2 weeks each month in summer. About once a month since last May, we’ve gotten a message from their mom saying how much they fight, physically and verbally. And that my step son acts out horribly and is “aggressive and angry”. They have 2 younger sisters that live there and he often babysits all 3 since he’s oldest, but only during longer visits, not weekends. This summer they have been SO amazing here. We had some issues beginning of the year but nothing major. We handle it here and deal with them. Overall they’re exceptional kids. I just had a baby and they’ve been so helpful. Literally no complaints at all. They went to their moms last Friday and not even after a couple hours there the mom texts us saying our son is speaking inappropriately and got grounded. Then the next day she said she’s so stressed out with how much they fight and she has to constantly separate them. They’re NEVER like this at our house! It’s been constant at their moms. We’ve talked to them many times and they both admit they get along poorly over there and fight a lot. Yet they also both said they want to spend more time there? So we sent them there for 2 weeks since their school doesn’t start until next month and that’s when she texted us with all the issues. We have no idea why they’re doing this or how to handle it. We’ve told her many times they arent like this with us and then she’ll just go silent about it. We’re definitely a much more stable home and environment, but that’s still no reason for them to act out so badly there.

EDIT:

Y’all I 100% agree with you on the why the heck is she texting us??? Lol! We have told her before that when they act out here we deal with it. If she finds out they’re grounded at our house she will flip out asking why and criticize us for it. It’s annoying. I do think it’s an attention thing. She is rarely home when they’re there, she’s out with her boyfriend most of the time or working. She always tells us we don’t know what we’re doing with parenting yet she’s the one always coming to us. 🙄