Omega Dean and Alpha Sam became orphans because of a housefire and was put in a facility/orphanage. Alpha Cas was looking for a mate at the facility/orphanage and saw Dean was nursing Sam. Cas wanted only Dean but, Dean told him they come as a package.

Cas only sexually involve with Dean and raising Sam part of obligation/promise with Dean. Dean continue to nurse Sam but Cas also suckle from Dean as part of sexual act.

At the end of the fic Sam go to school and Dean become pregnant. Sam still want to nurse from Dean but his chest was sore so he refused. Cas wanted to wean Sam off because we scared there won't be enough for his soon to be born baby.