General

I have a big problem, please help

Brief summary: mom wants to get divorced from a nice man who can actually take care of us, and i cant tell him the plans she has.

Longer version:
My mom wants to get a divorce from my step father of three years, and she told me about her plans this morning. I don't really get why she wants to have a divorce other than the fact that they fight a lot, but it's really her fault because of her crappy personality and bad parenting. And yes, even as her child I can say that she has a bad personality and is a bad parent.

My step dad is actually really nice and one of the coolest people i've ever met, and he's been putting up with all the crap she tosses at him without any qualms. Well, he gets frustrated from time to time, but he tries his best to keep the family together. Essentially, he's the only one keeping this household from falling apart. Not to mention he's the only one in this place that takes care of my three year old brother. Like, no joke.

The problem is that my mom said not to tell him, and it's kind of tearing me apart since he doesn't know it's coming. She wants to file for keeping the house as well as my brother and I, which I don't really agree with at all. My mother is in no way capable of taking care of me, let alone my brother. She makes enough income to sustain the home, but lacks in everything else.

Everything is going to happen within the next week or so, but i don't know if i should tell him or not. And is it possible for me to testify for my brother's caretaking, but not mine? I want him to live with the better parent in separation, even if it means that I'd have to stay with my mother because he isn't a legal guardian.

What should I do? Betray my mom's trust in me, or help the person that doesn't deserve all of this prepare for what's gonna go down? I'm sorry for the poor sentence structure, i wrote this while in a rush. And this isn't all of the information, there's a lot more. i just tried highlighting the important parts