Etymology From big sister + -ly (adjectival suffix).
1
Of or characteristic of older sisters.
Some were sweet and big-sisterly, good shoulders to cry on; others eternal cheerleaders, all freckles and bobby sox; still others were serious campus leaders with their blond hair in a severe twist.1988, Edmund White, chapter 4, in The Beautiful Room is Empty, New York: Vintage International, published 1994:
big-sisterly
adv
Etymology From big sister + -ly (adverbial suffix).
1
In the manner of an older sister.
By turns she could be goofy and serious, playful and big-sisterly stern.2015, Guy Vanderhaeghe, “Koenig & Company”, in Daddy Lenin and Other Stories, McClelland & Stewart: