Regardless sibling or not, if they were once separated, they will fight. How long or how bad they injure eavh other depends on their mentality. Strong mentality may mean a fight til half dead. But low mentality may end after a few bites.
You could keep them in separate tanks next to each other. Eventually one will stress. Then put them together. Remember to provide getaways not easily entered; like dense hornwort, anacharis, etc where they have to dash in to enter/go through. And make sure the getaways reach water surface.
Kept in a tank 13g or more (long type is better). After a while together, the alpha should calm down and they should fairly live peacefully.