Claim. Sibling love trains people to love the whole life of their parents, not only the parents as they appear in old age.
Elaboration. SMM says brothers and sisters are needed because boys learn from sisters how their mothers grew up, and girls learn from brothers how their fathers grew up (CSG 4.2 §4, 184-60, 1988.11.13). Loving siblings is “like watching and loving your mother and father when they were growing up”; it fulfills a condition of loving parents “from their birth to their death.” Parents also learn their spouse’s earlier life through sons and daughters. This makes sibling love more than companionship: it completes the family’s circle of love across time. The section’s practical rule is severe but clear: hating a brother or sister is more serious than hating one’s mother, because siblings represent the parents’ whole life.
See also. family