Are they all four brothers?

I would say 2 sets of twins from different mothers.

Mario and Luigi are bros  and so are the others but two different sets.

I don't think so.

they are kinda half brothers, only Luigi and Mario are from the same father, while wario and waluigi both have different fathers, but all of them have the same mother, Nintendo

It's about time that Waluigi should have his own game so we know more about his background and Nintendo needs to make another  Luigi's Mansion.

" It's about time that Waluigi should have his own game so we know more about his background and Nintendo needs to make another  Luigi's Mansion. "

Maybe? I don't know. Waluigi doesn't exist outside of the sports/party games though, so he's not really relevant in any way.

" @Alpha1 said:

" It's about time that Waluigi should have his own game so we know more about his background and Nintendo needs to make another  Luigi's Mansion. "

Waluigi's Mansion? "

Mario Bros, and Wario Bros. I think they're cousins, or that Wario/Waluigi came from a parallel universe. 

" I would say 2 sets of twins from different mothers. "

I thought about making a quick comic covered the entimology of all this but then I'd be one of those whore artists who makes a serious mario illustration for attention. 
 
 
 
Which I'm fine with. But I don't got the time right now. Quick spoiler; I manage to work in Booster.

Naw

You didn't see the Maury show drama explaining the issue?

waluigi and wario are brothers and cousins to mario and luigi which was stated in a nintendo power magizine which was owned by nintendo

Mario is Luigi's brother, which we all know from the Mario Bros game. You may wonder why it's called Mario Bros. Indeed, the green and red Italians (not coincidentally Italy's colors) are brothers, and they share the same last name. Their last name is Mario, hence the name Mario Bros. They are two "bros" in the Mario family. (Unfortunately for Mario, his name is Mario Mario). What's interesting is Mario was first a carpenter when he debuted in PS64. It has been determined since then that he is a plumber, as he often climbs underground in pipes. If you look closely at his outfit, he definitely looks like a plumber! So what about Wario and Waluigi? Well, Wario is Mario's "secret" twin. Evil twin, some might argue. The same is true about Waluigi and Luigi. Evil, secret twins. So would that make Waluigi and Mario siblings, and Wario and Luigi siblings. While, I haven't hopped into the game to check their birth certificates, and haven't been lucky enough to meet any of those mystical creatures (I want to hang out with some talking mushrooms real bad), it would seem pretty obviously that they are siblings. This makes some people wonder: Are all these characters siblings? The answer is no. Go check out the original story, where Toad and Peach are introduced in the castle scene. (Again, I haven't asked Mario or his brothers, but if I had to guess, Peach, Toad, and some of the other characters aren't siblings of Mario and Luigi). Lastly, Baby Mario? You all now what the steps of having a baby is. The most important part is-- WAIT DOES MARIO HAVE A WIFE?! Or is Baby Mario adopted? (Well, they look a lot alike, so I bet they're blood-related.) But does he have a wife? While, anything (including being married or having children whenever you want without a wife) is possible in a video game! I suppose next team I see Mario, I'll just have to ask him. I'll let you know what he says.

What did he say?