(I didn't mean the videos; I meant the people on this site.) Both sides are incorrect here.
There are plenty of ways to manage this matter and he shouldn't hit his sister, but if she won't listen, he might as well cut her out from his life. Honour killing or beating is not a part of Somali culture, unlike that of our eastern neighbours Yemen, Oman, Saudi Arabia, etc. (However, a small minority of Arabs practise this, but not a majority.) We shouldn't adopt this practise because it is against Islam itself .
I can see why Somali feminists are upset, but you can't blame Somali men or generalise about Somali males when it could happen to any race or ethnic group on the planet, and we wouldn't do that, would we?
In the end, I believe that the brother should receive the same punishment, which is being beaten in a manner similar to how he hit his sister.