Great!  So, to review your order:

biggest sister "Reagan" size youth medium (10-12)

bigger sister "Maggie" size youth small (6-8)

big sister "Ryann" size 5/6 (it runs bigger and it's pretty big on my 4 year old)

little brother "Seth" size 3 mos (it's carters brand and is there 0-3 size)