SE-SKOLORNAS 2019 Problem 1

The siblings Robb, Arya and Sansa have received seven sealed bags from an unknown donor with varying number of beads. Six of the bags have labels indicating the number beads: $7, 9, 11, 13, 15, 18$ , but the seventh bag lacks etiquette. The sensor has set certain requirements: Robb must have three bags and his sisters two bags each. In addition, Arya will have the bag that contains $7$ beads. The bags should be distributed so that each of the siblings get the same number of pearls (this is possible according to the donor). How many pearls are there in the bag without a label, how many pearls are there in total and how should the bags be distributed?