Are Waluigi and Wario brothers?
Are they related to Mario and Luigi?
It’s not really known, there are conflicting sources.
According to the Super Mario Wiki, Waluigi and Wario’s relationship has jumped between “brothers” to “just friends” and everything in between. When Waluigi was first announced in Mario Tennis, Wario was said to be his “big brother”, and several other sources at the time (including the Prima Games strategy guide for Mario Tennis and the Mario Kart DS website) said the same thing. However, some other games such as Mario and Sonic at the Olympic Winter Games have stated that Waluigi and Wario are only cousins. And still other sources (including Nintendo’s official Japanese bio page for Waluigi) say they have no relation at all and are simply two evil friends.
So in summary, we don’t know for sure. I like to think they are brothers, since it provides an interesting dark mirror of the Mario brothers, but it’s also quite possible their similar appearance is simply coincidence.