Step siblings don’t always get along

I (33F) have been with my fiancé (29M) for 5 years. We both have kids from prior marriages. I have a 12 year old son and he has two daughters that are 11 and 6. Our kids don’t always get along. My son is with his dad at the moment and was on Roblox earlier when my oldest SD (who is at her moms) was trying to talk to my son earlier on Roblox. Apparently my son was being rude to her and threatened to tattle on her about her talking to an old friend a Roblox that she’s not supposed to be in contact with. That’s about all the context I can give. But my SD went to her mom and was crying about my son being rude to her so her mom called my fiance and full on cussed him about about my son’s behavior. My fiance was calm and explained that they go back and forth with eachother and that all the kids can be rude to eachother. Oh, but we’re not going to talk about the oldest daughter is still sneaking talking to the friend she was told MONTHS ago should could not talk to or hang out with anymore?? But, I’ve witnessed my fiancé’s daughters being so rude to eachother as well as their cousin (on their moms side) who we’ve had stay over at our house before. But I feel like when it’s my son, the tables flip and my fiancé gets so angry because he doesn’t want his girls mom to call him bitching about my son. Like my son is the only one that really gets in trouble because of their mom. She has been a HCBM in the past and their divorce was nasty and my fiancé just says that he doesn’t want there to be any reason why she might try and hold the girls over his head. I’m just so lost. Kids are freaking kids. My siblings and I fought like crazy growing up. My fiance has told me stories about how mean he was to his younger brothers growing up. So what the hell? Why is MY child being singled out? His behavior is NOT excusable by any means. But this just doesn’t seem fair.