We were best friends from the age of 5 until we were 18-19 years old. Him, his brother, my sister, and myself. We were inseparable as children. Even though we went to different schools, and had different sets of friends, we would still see each other weekly, and spend a ridiculous amount of time together in the summers.
I'm not sure what happened, but my sister and I grew apart and fought a lot as siblings do when the teenage angst really kicks in, and he started to gravitate toward her more than me, I thought nothing of it, they weren't romantically involved, and I would still be over his house constantly. Then one day, he started prodding me about my personal life, and what I'm doing with my girlfriend, etc. etc. This rubbed me the wrong way, because it was so unlike him, and I finally got it out of him that he was trying to milk me for information because my sister asked him. He then tried to barter with me for information, using dirt he knew on my sister. I told him no, and he dropped it. But that was a red flag, and I was always suspicious whenever we would talk about our personal lives.
Fast forward a few years, and he moved away to texas to be with a girl he had been dating for a month at the time, I told him to think about it, but I would still be around if he wanted to talk, and if he ever moved back. Well two years, and a failed relationship later, he came back. But things never were the same. He spent all his time with my sister, and whenever I would ask if he wanted to hang with me and his brother (we became extremely close in the interim) he would say no to us, and then bitch to my sister about how I'm constantly blowing him off. She would yell at me until one day, instead of asking in person, I asked him over text to chill, he said no, and when my sister confronted me, I showed her the text. She stopped hassling me. Red flag number two.
The final straw came when we were around the age of 20. In a last ditch effort, he invited me over, and I said I had some weed on me and if he provided the beer, I would smoke him out and we could finally, formally, catch up. The night went extremely well, we got a few friends together and it was like old times. And then he stopped texting me.
Fast forward a year later, I had moved to texas, and my mom had found out I was smoking. I asked her how she found out and she said my sister had told her, because that night, he had taken a picture of me, and sent it to her saying that I'm a giant pothead.
I haven't spoken to him since. I should have cut contact before, but I was hoping against logic that things would go back to normal, because I missed my friend.
TL;DR don't ignore red flags, even if they're with who you consider to be your closest friend.