Male bettas prefer to have 3ft square territories, why when you see 2 male bettas in a tank together they tend to be in a 55g+ tank that is heavily planted.
Being siblings does not guarantee they will get along - ask any breeder, 99% of the time the males will be pulled from the grow out tank for being too aggressive with each other and need to be separated. It is rare to see sibling males living their whole lives together - the only person I know who does it successfully is Oldfishlady after multiple generations and lots of work.
Females tend to work better together if they are siblings, but again it depends on each one, as they each have their own preference.
Basically, to house two sibling males you will require a large (close to 55g and bigger), very heavily planted tank to even attempt - a 10 gallon is not going to cut it at all. And even with the best set up there is no guarantee they can live semi-peacefully together... it really is not worth the risk to place them in a 10g together. The only alternative that is safe and recommended is to divide the tank so each male can live in it safely.
Breeder so I have too many to count and too many named to list here!