Mystique gets sent to court for tax evasion and the brotherhood need to defend her in court. This doesn't make any sense legally. But I can do what I want.
A hill I will die on is the fact that the brotherhood were an amazing concept but there potential was wasted. I would have loved to see more of them but unfortunately they were only used when convenient for the plot.
I know for a fact that Adira falls into this, and I suspect the rest of the Brotherhood do, too: I was told by crew that they didn't know what to do with Adira. That they invented this cool character to fill a specific niche, but didn't know what else to have her do. That's why she was so in-and-out of the traveling party. That was why Cassandra's animosity towards her ended up being an animosity towards Rapunzel instead. And I suspect -- knowing what we know about the showrunner -- that dear CS was scared that people would like her more than Cassandra, just like how people liked Varian more. (Not everyone, obviously, but that he got such a big and loud fanbase clearly didn't sit well with Mr. Executive Producer.)
I also suspect that, if they didn't know what to do with Adira, they sure as hell didn't know what to do with the rest of the Brotherhood. That's why the Mind Trap is such a weird and poorly thought-out concept. (I maintain that the fact that a mind control device can be handed over to someone under its influence is insidious, but also backwards.)
Anyway, I'm not nearly as into the Brotherhood as a lot of the people who are active in the tags, but I agree that they're an amazing concept with wasted potential. There also should be more than four of them.
And also, unpopular opinion, but Varian isn't a part of the Brotherhood, just by association. He's got nothing to do with it. Neither does Eugene. Just because their dads are indoctrinated doesn't mean that they're going to follow suit. Neither one of them knew anything about it before the events of the series, and I somehow doubt they'd look at the Brotherhood now and say, "Yeah, I want to be a part of that!"