sisterly

adj

1. Of or characteristic of sisters.

   “Poor Mary [Austin] will do her very best and sisterliest for you. A kinder soul is not in the Earth.”

   “Burton designed both Middleton sisters' dresses in white, so that when Pippa picked up Kate's train in the abbey, the two of them were united, visually, in a display of sisterly togetherness.”

adv

1. In the manner of a sister, behaving as one would expect of a sister; as a sister, as sisters.

   “"What honest man, who is not insane, would take lost women and thieves into his house to dwell with him sisterly and brotherly?"”