I'm called Mark, Matthew and Jack. None of these are my name yet I don't get in a huff over a mistake. It happens, especially with siblings. You're only mixing up her name to be petty and vindictive. Some people laugh when they get nervous or make a mistake, it doesn't indicate that she's doing it intentionally. Or she might be laughing at how big a baby you become over this non-issue and wowee, I'd probably snicker a bit if I saw someone your age getting their undies in a twist over this.

Now maybe she started to do it intentionally because it's honestly funny and ridicilous how upset over this in which case it you'd both suck. I don't think that's the case and the only reason you're getting so many not TA votes is because this sub loves to hate on step mothers because everyone here is an expert at projection. If it really is intentional, be the bigger person. You're 15, mate. Grow a pair, YTA.