You can house siblings of different sexes as long as they are divided/separated.. it takes many years and generations to keep males/females together in very large tanks (speak to Oldfishlady as she successfully keeps them that way). But just buying a sibling pair won't mean they will be fine - ask any breeder and they will tell you just how long keeping siblings together lasts.. that usually tends to last a short while before needing to be separated.
Some are more gentle when they feel secure in knowing their neighbor can't get to them.. so they don't feel the need to show off or fight for their territory. I have some that are very gentle like that, yet others who will flare at shadows. Got two sibling males yesterday from a breeder who won't stop flaring at one another.
So it's a toss up on what personality you get.. but you can bet that putting two males together in a tank without a divider, regardless of relations, you're going to end up with two injured, unhappy, stressed fish.
Sorry.. seen a few posts today so far about putting sibling males together and it worries me that some think it will be fine - truth is, there are certain circumstances where you can attempt it, but even with the best set up it's not guaranteed and not worth the risk, imo. And I will never say it's a good idea to place males/females together unless they are divided. Again, under certain settings one can attempt this, but for the average keeper and in anything under 55g, it's highly not recommended.
Think for the safety of the fish, and their natural instincts rather than what we would like them to be. If they were like any other fish that can live peacefully together, then they may not be the personable fish that we came to fall in love with. Would be like all other fish who don't pay us no mind.
Breeder so I have too many to count and too many named to list here!