D&C 124

The Lord revealed in the early days of the Church - 

"...that my servant Hyrum may take the office of Priesthood and Patriarch, which was appointed unto him by his father, by blessing and also by right;

92 That from henceforth he shall hold the keys of the patriarchal blessings upon the heads of all my people..."  (Doctrine and Covenants 124:91–92)

Also - 

"First, I give unto you Hyrum Smith to be a patriarch unto you, to hold the sealing blessings of my church, even the Holy Spirit of promise, whereby ye are sealed up unto the day of redemption..."  (Doctrine and Covenants 124:124)

As Church government has evolved, where there is no Church Patriarch, I believe the Quorum of the Twelve Apostles holds these keys of patriarchal blessings under the direction of the First Presidency.