IOTL, he practically killed Sumner as things stood. Sumner had to take a leave of absence from Congress for six months or something ridiculously long since he was so badly injured (iirc he went blind in one eye or something and walked with a pretty bad limp forever after).
But yeah, Brooke is definitely going to spend some time in the slammer and I wouldn't be surprised if he got sent home from DC afterwards. Not sure who'd take his place, but I assume there's a contingency for that. I'm not sure it'd affect the going-ons in Congress that much though. At this stage IOTL, shit was hardly getting done anyway on account of the gridlock, so I figure it can't get much worse than IOTL.
On the other hand, if Sumner's dead and Massachusetts can immediately appoint a successor to his post, his seat won't go unfilled as IOTL when they just had to leave it empty while he convalesced (I think it worked that way, don't remember). Maybe there's some crucial legislature his successor could vote on during that time period or something.