Hi, I am looking for some advice and some examples if possible

I have a ss11 and ss8 and a daughter with my partner who is 1yo. For the last 12 months, the step sons have been in our primary care, with their dad a stay at home father. They see their mum every second weekend and when she feels like taking them out for dinner during the week.

We have worked really hard to put structure and stability in their lives which was missing when they lived with their mother.

Last night ss11 had a pre-teen tantrum when he didn't get ice cream but his brother did because he didn't eat any vegetables and stormed to his room. He got on the phone with his mother and organised to go to her place the next day (a day earlier than agreed). He came and told his dad, who said there would be no early swap. After talking with his mother, she begrudgingly agreed.

SS11 denies he wanted to go to his mothers because he was upset and angry. He says he needs to spend more time with her. And that we are "keeping him from his mother" and making him very upset.

Does anyone have any suggestions what to do? I worry that if we start this now, it will never end "I don't like your rules, I will go to mums place".