Like the Catholic church, but unlike some other religion, the Mormon church has a very clearly-defined organization with a clearly-defined leadership structure, and a clearly-defined set of established doctrines and practices. The exact history of the FLDS is unclear, but apparently some time in or around 1913 a small group of people had disagreements with the Mormon church, it's leaders, and some of the practices and policies of that time, and so they broke off and started a number of other churches, one of which developed into what is now the FLDS. Like the Lutherans, they rejected the Mormon organization, the Mormon leaders, and some essential Mormon doctrines and practices, in order to form a new religion. The FLDS and any similar organizations which may exist today are not part of the Mormon organization. They reject the Mormon prophet, the Mormon leaders, the Mormon organization, and many essential Mormon doctrine and practices. They are not Mormons any more than the Lutherans are Catholics.
There are some religions which are vague enough in structure, organization, leadership, and doctrine that it is possible for some to drift from the historical base of that religion, form new teachings and doctrines and organizational structures that are not part of the larger body of that religion, and still reasonably call themselves by the name of the parent religion and consider themselves still to be a part thereof. Mormonism is not such a religion. Mormonism has a very clearly established organization, structure, leadership, and set of doctrines and practices; and any group that breaks away from that (as several have) is no longer Mormon.