If siblings grew up apart, never knowing (not in the biblical sense) each other until they met in the same manner strangers would, would the absence of the knowledge of their kinship make it a morally unacceptable? I'd say not.
The problem I see arises when siblings are brought up together. In every family there are degrees of power and influence. An older sibling may pressure a younger sibling, which can have other consequences, such as the younger developing a warped attitude towards the pressure and power-play in that relationship and in their life in general.
Other than that, from my experience with my siblings, we rarely sat down at the dinner-table without starting a fight, which I think is the way it should be.