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.
In the manner of a sister, behaving as one would expect of a sister; as a sister, as sisters.
1907, Jack London, The Iron Heel:
"What honest man, who is not insane, would take lost women and thieves into his house to dwell with him sisterly and brotherly?"