I understand that brothers and sisters will argue but they do it all the time in fact I can't remember an episode where they didn't argue over something stupid. Adam and Chase argued on Lab Rats but it wasn't in every episode and it didn't dominate the episode. You can have conflict between two characters without having childish arguments.
shareI think their bickering is cute and it's not usually fueled by hate. Max said they care about each other - they just do it because it's fun.
Also, not every family has a sibling who wants to become a supervillain. Considering that Max was being coached by an evil bunny to become a villain for years, it's not surprising he would try to put down his sister any chance he gets. And at the same time, Phoebe couldn't fully trust him or any of his plans because most were evil or pranks. So, that inevitably led to frequent fights.
Now, even though Max is no longer trying to be evil, it's not easy to just stop being mean to someone you used to fight. So, it's going to take time before they both learn to trust each other.
That said, the fights are quite superficial because the twins always end up working together. They pretty much spend all of their time together. This is one of the few shows where a brother and sister share almost all storylines and scenes. So, do they really hate each other?
It's better than the co-dependent relationship that Billy and Nora have.
https://www.fanfiction.net/s/11891397/1/Supernatural-Wayward-Sons