This is something where the difference between an advice column and a conversation with a friend or a therapist stands out: either of those people would have useful background, and be able to ask questions about things like LW's relationship with their brother. Like, does "unexpectedly to me" mean that they hadn't talked to any of the people involved in a while, and it's like learning from a Cbristmas letter that one of your cousins is moving to Oregon, or does it mean that they'd been in touch, but it went from "I'm annoyed that Mom did X" to "I'm no longer talking to them" quickly and without LW knowing why?

One significant piece of the puzzle here is who told LW about the break: one or both parents, or the brother? (I'm guessing the parents, but I may be reading too much into the word "sharply.") Any advice I might give the LW would depend significantly on who LW heard this from, and on what sort of relationship, if any, they had with their brother.