I don't think it is right to have two closely related persons working in the same organization. It makes staff harder to manage as you would always have a multiplier effect of an action taken on one person affect the other.
If two sisters are working for me, and something happens and I punish one of the sisters, it is going to affect the other sister too.
If two sisters are working for me, and something happens and I punish one of the sisters, it is going to affect the other sister too.