You sound like an obnoxious dumbass kid who throws around buzzwords they don't understand. Sister sounds pretty dumb too.

Saying "Sorry I'm a piece of shit" seeks out sympathy to override accountability. I can't stand people who do that.

“and I told her I was sorry for being a piece of shit,” Man, this really is a “the thief thinks everyone else is a thief” situation with you accusing your sister of gaslighting, huh. That was not an apology, bud. That was a manipulation tactic to make your sister feel bad and drop her criticism of you. I can’t speak to the veracity of her claims about her mother, but at the very least it is something plausible and upsetting enough to her that she wanted to discuss it with someone she trusts. And rather than letting her vent or just saying outright that you’re uncomfortable discussing this, you apparently chose to be a jerk about it.

Sounds like you know something you don't want to tell your sister. At some point the truth will need to come out. Maybe your sister isn't ready to hear it now (maturity, social or mental stability issues), but keeping it a family secret can't last forever. And it could cause other issues if not aired out and used to help others prevent the same fate (mental health or drug issues).