New Member Alert!

I'm a twin (both female and identical). We've never formally diagnosed with BPD so I could be blowing things out of proportion. Although I have always seen BPD symptoms and characteristics in my twin sister. However, it could easily be me gaslighting or overreacting... My family and I do not currently have the time or financial resources to diagnose each other with mental illness. Although, I would not be here if arguments (over random things like commercials) did not have a 1 in 4 chance of devolving into violence.
Even though we did not fistfight this time I was inspired to search online for tips on how to get through disagreements or differences in opinion after we fought about some random woman in a commercial. I'm 20 years old and I just want to be able to not get into screaming matches over random small things. Every time it happens I feel like I'm behaving like a 5-year-old.
While reading some of the articles on bpdfamily, I noticed some things about myself. I always prided myself on being a "stand for what's right" kind of girl. It's one of the only favorite characteristics about myself. I just wondered
what are some mantras or steps you go through to prevent yourself from saying something. In my case, I think my twin, in some instances, is cruelly cynical and intensely hypercritical. We'd be watching a commercial and all of sudden she would start antagonizing mother, child or father on-screen. How do you go about holding your tongue? What are the ways you resist the urge to telling someone to be a little more openminded or empathetic? Even though I could try working on my phrasing, but I can't imagine a way of saying something like this without her perceiving it as a passive-aggressive jab.
Even writing about has already helped quite a bit... I realize becoming a martyr for a fictional family is probably not worth the trouble. I'll keep this in mind next time. Although, I can't foresee myself fully resisting the urge to say something if she does something similar again. I guess I'm worried complacency with her negative opinions will lead to her openly insulting and hurting our family members and friends.
For example, one of my cousins recently got in trouble with the law. He's still a teen though, but he's somewhat proud of his
street cred. While I understand the need to discourage his behavior when he's being a prick about it. She stoops to cussing him out or low-blow insults (even in public and in the presence of strangers). In these situations, where I feel she has no place to insult him and it's not the time or the place what should I do? I know what articles are telling me to do, but I want to understand what real people do to avoid arguments.
Thanks for listening! Sorry about the self-righteous monologue! Especially, if this wasn't even the way you are supposed to introduce ourselves. Sorry in advance!