Yes, I had problems with a step sibling growing up. I lost my Mum when I was 3 and her Mum (my Nana) aged 10 and I think that hit especially hard age 10- at the same time I was having all these troubles with the step sibling. I guess I was looking for comfort and you look to your Mum especially for that I suppose.
Without going into too many details, she was very verbally abusive but she also accused me of a whole load of things I hadn't done. To the extent, I got pulled up in front of the deputy head master. (May not sound like a big deal but I was a very quiet, shy child- it was awful).
Kind of the last straw, she ripped up a piece of my coursework and then accused me of stealing money from her (to deflect blame I assume- stupid part is- I didn't work that bit out for years- I was in shock I suppose. Dissapointing part is- my parents just went along with the narrative). We ended up with locks on our doors. Naturally, all this caused a major rift in the family.
I know this is going to sound trifling to some of the abuse people have endured (and my heart breaks for them) but it was still enough to make me first consider ctb.
Honestly, I was really scared of her- of what she might do next. I was so happy to leave home to go to uni.
Kind of recently, I started watching YouTube videos on narcissism and was kind of amazed to see all of those behaviours described. (The accusing someone of things to make themselves out to be the victim, the idea of 'flying monkeys'- people who support the narcissist.)
I know I have many flaws too. Not saying I'm totally innocent- I've done things I regret (although ironically- not to the step sibling- she was far too intimidating for that). Still, it kind of helped me to know there was a term for this type of behaviour- even though it's only one I can diagnose her with in my head. Makes it all kind of less crazy when you know there are terms for things! Not that it erases anything.
Don't think my fascination with wanting to work out why things might have happened has much to do with ctb. Doesn't make me want to do it less but suppose it did make me feel slightly more validated in a way. Think I spent a lot of time just feeling scared and confused.