In order that there may be no misunderstanding in regard to the matter of the sealing in the temple of children to persons other than natural parents, we are pleased to give you the following information for your guidance. Children who have not been born in the covenant or sealed to their natural parents may be sealed to a natural parent and stepparent. ... Children born in the covenant cannot be sealed to persons other than their natural parents. This rule is not altered by adoption, consent of the natural parents, or request of the child after becoming of age. Illegitimate children may be sealed to their natural mother and the husband to whom she is sealed without the consent of the natural father, and only the natural mother's consent is necessary for illegitimate children to be sealed to foster parents. No sealing of children to persons other than natural parents will be permitted except as set forth above without written authorization of the First Presidency.
[David O. McKay, Hugh B. Brown, N. Eldon Tanner, and Joseph Fielding Smith, circular letter, Jan. 8, 1966, in Anderson, Devery; The Development of LDS Temple Worship, 1846-2000: A Documentary History, http://amzn.to/TempleWorship]