I'm NB and I used emailing to come out to my dad and writing a letter for my mum. It helps a lot when it's in written form since it means you won't forget anything or something like that.
I do think your brother is capable of understanding what it's like being transmasc and I also think that your mother, even though she's known you for a long time, still has to put an effort in naming and gendering you correctly. Though if she's trying and keeps forgetting, it's pretty different to her not really caring and using having known you for a long time as an excuse. Intentions matter and whatnot. Like my 15yold sister was misgendering me and deadnaming me two weeks after I came out, not out of bad intentions but since she was just a little bit naive and when I reminded her, she started putting in a lot of effort, and supported me.
If you're worried that your brother is too young to understand, perhaps try finding some resources online to give to him.
Though good luck with it all, hope it goes well :)