Timing can ruin your flow. Focus isn't the only thing here. Brothers barge in at bad moments. It's hard to keep your head in the game. Blaming focus alone misses the real issue. Both timing and concentration need work.
Spencer saying "both timing and concentration need work" is right but it's deeper than that. The real problem is the UNSPOKEN RULES of a shared space. Your brother thought it was just a room, but you were in a zone where a second costs money. That mismatch is what causes the scare, not just the bad timing itself. You need a clear signal that means "do not cross this line right now," and everyone at home has to respect it.