I'm not sure on any "formal" etiquette but it's differed at funerals that I have been too.
Given there are four siblings and if there is not much space at the front of the church, I'd have said they should all sit up front with spouses behind (also with any children).
Same as ummmm, most of the funerals I have been to the family and their spouses can take up to the first 4 rows. The spouses are normally sitting with their partners.