It can sometimes be third party sh*t stirring.
My best friend stopped talking to me in 2012 and a few years later I asked her 18 yr old son (my godson), if he knew why. ( I waited until he was an adult before asking him out of respect for my friend).
He told me that I'd said something about her but the nasty words he repeated were the exact words that a different friend had said to me, about her, several times. I had disagreed with the negative crap that this person had said and so I'm guessing that this person ran back to my best friend and told her that I said it. Ironically my policy is to never repeat crap to my friends so that they aren't affected by negative crap and so I would never have run back and told her what the 3rd, "friend" said.
Some people just never leave high school and sometimes it's your own sister.

Everyone makes bad friends sometimes, you just have to learn to spot them and weed them out faster, unless it's your sister!