It was subtle and took years, Pina. My parents had pets, which he couldn't spend time near (he now has one of his own - twat), they didn't do Christmas right, they didn't even believe in it properly (his mum went to church once a year, his dad, never). I became an expectation to spend a couple of weeks at a time staying with his parents, but somehow, there was never time to go see mine (we were several hours away from either set of parents).
He constantly took the piss out of one of my younger siblings, had tantrums when playing a board game that we beat him at more than once, took the piss out of them some more....
Interestingly, when we stayed with his parents, he reverted right back to teenager mode, even at nearly 30. The way he and his brother talked to his parents was quite disgusting, sometimes.
It was only when my sibling got married without me even knowing about it, years down the line, that I realised how big a rift had been created. I was already feeling pissed off about other stuff - lots of subtle but real EA and generally being sick and tired of all the drama around him - that I thought stuff it, drew a line, made a point of visiting my family alone and getting to know them again and refusing to spend more than 2-3 nights staying with his. I was a fucking grown up with a fucking full time job and owned my own fucking home, ffs! Tough shit if he didn't like it!