Family Merger can only be bought with family points. It can be used by F=family master or family administrators. The family that the family master wants to merge with have to agree to the merger. Once used, the item will disappear from inventory.
The family that invites other family to merge will take over the control of the invited family. All of the members which have their family merged will be Associate members of the family who used the merger, while the master will be an administrator.