JOEL - LDS do not consider animals as brothers and sisters but do believe that God has a purpose and a plan for thier existance.
God gave Adam and Eve dominion over the animals (Gen. 1:28) and allows the eating of animal flesh but forbids its waste (Gen. 9:2-5; D&C 49:18-21).
Animals, like other "good things which come of the earth, are made for the benefit and the use of man," but are "to be used, with judgment, not to excess, neither by extortion" (D&C 59:16-20).

We believe that all living things, both humans and animals are composed of both body and spirit.
In one of our scriptures(Doctrine and Covenants 77:2) we learn:

Q. "What are we to understand by the four beasts, spoken of in the same verse(Rev. 4:6)?
A. They are figurative expressions, used by the Revelator, John, in describing heaven, the paradise of God, the happiness of man, and of beasts, and of creeping things, and of the fowls of the air; that which is spiritual being in the likeness of that which is temporal; and that which is temporal in the likeness of that which is spiritual; the spirit, of man in the likeness of his person, as also the spirit of the beast, and every other creature which God has created."

And in Moses 3:5 we read:
"And every plant of the field before it was in the earth, and every herb of the field before it grew. For I, the Lord God, created all things, of which I have spoken, spiritually, before they were naturally upon the face of the earth"

As explained in these scripture, like humans, animals were created first as spirits in heaven and then physically on the earth. So it wasn't a matter of deciding which spirit would be an animal or human. From the begining God made human spirits and He made animal spirits and He places them in their respective human bodies and animal bodies.
According to Joseph Smith, the salvation of animals is covered by the atonement of Christ. They will die, be resurrected, and be placed in some level of heaven(Teachings of the Prophet Joseph Smith, pp. 291-292).