Our last little tribe of 5 were very close. The oldest two were a year apart and very much "Mum & Dad" to the younger 3 boys (one 2 years younger than "Mum" and 2 years older than the youngest two, who were brothers) - they were quite protective over them at times, and less bothered by their antics at others. When we lost one of the two litter-mates, "Mum & Dad" pined for a while... then threw themselves into being around the remaining two (who had been plagued by the deceased twin something chronic as they grew up - he was 3 when he died from a calcium block in his urethra, so very much a teenage thug, whilst the other two were more placid and easy-going). "Mum" died, next and "Dad" stepped up (I should probably add that "Mum & Dad" were adoptive parents, not biological - all 5 were foster kittens who never left!) for a few years, during which the two boys looked after him more than he did them. Especially the remaining twin. As "Dad" aged (he was 22 when he died), the youngest boy (who was 11 when we lost "Dad") became akin to his carer - he'd sleep between "Dad" and the edge of the bed/sofa, he'd stand next to him whilst he ate, he'd just generally follow him around, where the older boy didn't. At all. We lost the two remaining boys last year, 4 months apart. The remaining twin was the last survivor and he became quite depressed towards the end - not eased by the arrival of two kittens in our midst, sadly. He was 13 when we lost him, unexpectedly, to a saddle thrombosis, a year ago tomorrow.
His loss upset the little boy kitten, who strove to be just like him... and sort of is. He and his sister aren't close in the slightest, sadly. To the point where, the girl is going to leave home when my daughter moves out... because I know she'll flourish as an "only" cat. The boy kitten has my dog for company for most of the day - he adores my dog to the point where I'm pretty sure he thinks she's his actual "Mum", and he's part-dog. As I'm typing this, they're curled up in a heap together on the end of my bed, boy kitten cuddled up to his dog Mama's belly, snoring their heads off. Girl kitten, meanwhile is probably still hiding beneath my son's bed, waiting for him to come home from college...
I have no clue as to why our first tribe were so close... and the brother/sister duo aren't.