I am a cutter too. I cut almost 2 weeks ago while sober and trying to get through some withdrawals. I gave up wine long ago (didn't drink much then) but found I would get weepy and cut while drunk. From my own experience, cutting is not usually about attention seeking, but more from needing a release from the hell of intense emotional feelings I am feeling. I regret it later but it usually makes me feel better in the short term.
I have my own opinion on why the mechanism of cutting works short term but have no medical experience so I will not share that now.
PS I have never wanted to cut anyone else, only myself.
Good luck to your brother and also you.
~Kricket