Depends on the fish (seems I'm saying that a lot today). We have one tank of turqs born in early Feb. who have all lived together (boys and girls) quite peacefully--including 4 babies from another spawn from the same time. The boys were only separated this weekend to encourage their fins to grow out (have no scientific data but someone thought that the dominant males may release hormones into the water delaying the maturation of other males in the tank). Of course, this is the most peaceful spawn we've ever had. The boys from the April spawn were also jarred this past weekend because they were nipping everyone but they've been very aggressive since they were big enough to see with the naked eye. So, it really depends on the fishes. In general, most sisters can remain together without much trouble--and they'll tolerate one brother in among them as long as he doesn't get uppity (they'll gang up on him and tear him apart if he does).