My brother is my best mate, I cannot imagine a life without him and I am certainly enriched for his being around. We tend to come as a pair despite there being 5 years ish between us. We do a lot together and our other halves have o understand that pretty quickly.
Not sure if it promotes responsibility or whatever. I am inclined to think owning a dog has a larger impact. Tell ye what, though - when it comes to bands, me and him and properly in tune and we never have to second-guess each other. It's weird. We are totally in step, or can be with minimal effort.
I am glad for him and we have been (and indeed can be) a fearsome combination. We are well known round here amd not always for the best reasons.
I have family (actually an unusual amount - 3 sets that I can think of!) who are twins but no twin mates. I do have mates who are close brothers like us. It's very good.