DaniLeigh's Brother Brandon Bills

DaBaby was allegedly assaulted by his baby mama DaniLeigh, and now her brother wants to beat the crap out of him ... legally, of course.

We got Brandon Bills at The Grove in Los Angeles and talked to him about challenging his sister's ex to a celebrity boxing match.

Brandon, who's also a rapper himself, says he would win handily ... guaranteeing the fight would end with DaBaby out cold on the canvas.

As we reported ... DaBaby and DaniLeigh got into a blowout argument this week in North Carolina, and cops later charged her with 2 counts of simple assault after police made 2 visits to DaBaby's home.

DaBaby also posted a video where he seemed to accuse her of using their baby for clout, vowing to…