It's supposed to be Коалиция Сириуса. The other way around simply hurts slav brains because Сириуса denotes meaning similar to "of/from Sirius" so instead of saying "Coalition of Sirius" you get "of Sirius Coalition". If you want Sirius to be the first word then Сириуская would fit better as in that form it's an actual adjective that can be used first.