NTA, but you [& I know subs like this can jump the gun on this] need her out of your life ASAP. She's acting like a petty child ["So & so doesn't wanna talk to me ? FINE I HATE THEM & DONT WANNA TALK TO THEM ANYWAY"] instead of realizing he's [probably] depressed & might be dealing w/ other issues that have nothing to do w/ her, as well as putting the blame on a child for an issue SHE caused. I know you said you need space from her, so hopefully this was already something you were thinking about, but still.

I'd also suggest family therapy ? For you & your kids, not stepmom, because trying to pit siblings against each other, excluding a child for not being ideal for her, blowing up because you love your son & won't exclude him, etc etc is beyond evil & childish, so I believe that that behavior is beyond repair [& even if you could fix it w/ therapy, it'd just put both of them - particularly Leo but both, in a terrible environment emotionally, cause those aren't issues that can be fixed over night]