Forgive me if im dense on this..
brother in law beats wife... so basically its your wife's brother yes?
Either way, dont lose your head and do nothing stupid. Punch him to the ground then call the cops. They will understand why you punched his lights out. They wont understand if you use a baseball bat though... besides, fists are so much more satisfying to beat those cowardly salamanders.
Then teach your wife some martial arts and buy her a tazer. Tell her it looks "chic" on her.
Of course, you wont want argue with her after that... a tazer in the nuts aint nice..
