Luckily, though an only child, I had plenty of friends with siblings and I quickly caught on to that 'sis' or 'bro' was never used in anything except the most facetious fashion.
Me and my sister call each other “trash” (sounds better in our language). We constantly make jokes about the other being trash (I.e “I gotta take the trash out” with a subsequent “why do you need to go out?”) etc.
My bro used to call me shitpants (in German) and I callt my cousin lovingly "little shitpants"
Also been called clevershit a lot by my siblings (which was fair ig.) But when things got tough they had my back.
i have a lot of siblings, and the only time ive ever called any of them "sis" or "bro" is when my older step brother and i make fun of the "step bro" memes
Since we're taking a poll in these comments, my go tos for my brother are "Hey Brotato" and "Hey Broseph", while my sister gets called her shortened first name (which she doesn't even go by, and hates, haha). Only time she gets sis is if I'm feeling silly and yell "Oi Sis!!!" with a vague cockney or pirate accent for some reason.
My brother and I refer to each other simply as 'brother' and always without fail do the 'brothers gotta hug' line from Tommy Boy. He's a CW5 in the Army but doesn't give a shit and it raises eyebrows, why we do it.
My sister, I used to call her 'sis' but like a decade ago she met my brothers twin kids for the first time (he was stationed in Hawaii), and my brother tried introducing them by saying 'do you see that lady, she's your aunt' and they immediately shouted 'HEY LADY', asking for something or other, so now everyone calls her 'lady'
Edit: my mom has a photo of my brother rubbing (then) Vice President Joe Biden's head 'for good luck' in Afghanistan back in 2009 or 2010, can't make that shit up. He shoulda been a comedian but instead he's doing redacted stuff
I have two sisters and I've called them "sis (insert sister name)" since so young that when I was about 4 someone asked my sister's name and I said "sis Karol" as if sis was part of her name lol. Only one note, I said "mana" instead of sis, I translated it to sis here because it is pretty much the same meaning in all ways