My brother and I don't always talk, and have sometimes gone years without seeing each other. When we are together, we get along great. When I listen to myself on video recordings, I recognize intonations and facial expressions that we both have. We are the same height, both have PhDs, though in vastly different fields. (His is quantitative, and he works in private consulting, after some years in academia and government.) He is skinnier than I am, though that used to be the reverse.
We have some similar hobbies. We began to play piano without talking to each other about it, within a few years of each other. He plays Goldberg Variations; I play jazz and Mompou and Chopin. He started birdwatching recently; I started a year a go. He has an expensive camera to take bird pictures; I don't take many pictures, and only have my phone as camera. (We went on my last trip to DC [this week] to a place where there were lots of cedar waxwings and other song and aquatic birds.
We both run, though I haven't been running recently, and he has done a marathon this year. We both like Samuel Beckett.
I left the church early; he has lapsed more recently. None of our daughters is religious. One of his, Chris, is about the same age as my Julia.
In short, it is like having someone who is different enough to be contrasting in some ways, but with underlying commonalities. I know this sounds rather banal to point out, since that is the expected result of two people of the same gender sharing common genetics and upbringing.