The problem is likely that since you intend to add the sibling from the mutual parent's profile, while still having the original sibling involved in an edit could cause the conflict. After all, the new sibling will be added as not only a child but as the new sibling which is being edited. Solution would be to save the sibling as is, add sibling (even from father is OK) and then return to the profile of which you were working). The new sibling won't display yet until you close and reopen it. Otherwise, the sibling is there in the background and after you finish your edit, close and reopen and the sibling will display. Good luck!