I don't know what to do about my little sister.

This isn't a vent nor a rant. I am genuinely concerned about her. Our relationship has improved a lot over a few months and we tend to stop arguing with each other. I tried to enforce my big sister/sibling privilege on her so she can vent, rant, chat, talk to me about anything; get things off her chest. It worked.

So, here's my concern.
There's a pregnant 17yo (about to be 18 next month) and she's about to have her baby next month. She's been picking fights at school as well. Her best friend had to move schools because they had a fall-out, which caused them to physically fight. Her best friend didn't like the drama, so she moved schools. (Moving schools, as in mean moving to another city).
On Monday, this week, the 17yo came at my 14yo sister, fought her, because someone was was instigating, telling the 17yo that there's a Tik Tok of my sister talking shit about her. My sister vented to me about what happened at school, in the cafeteria. She told me that in the middle of the fist fight, the 17yo blacked/passed out. Now the 17yo is going around, telling everyone that my sister kicked her in the stomach.

Weeks before this even occured, my sister told me about if the 17yo starts a fight with her again, she'll kick her in her stomach and kill her baby. I told her the consequences of her would-be actions and she said she didn't care. I asked her if it'd be worth it, and she told me, "Yes." Cannot convince her otherwise. I didn't get mad at her and tried to lecture her about how serious her conspiracy was and how she could go to juvenile for that, because we both are just now getting along without any fights and heated arguments.
When she was telling me how the fight went in the cafeteria, I was holding my breath, waiting for the part where my sister tells me that she kicked the 17yo's belly. That part never came and I was relieved.

Both me and my sister agreed that the 17yo should be the bigger person and ignore any instigators that'll rile her up to fight. She's pregnant, for crying out loud. Not only that, she's going to be 18 next month.
I know I may sound like a hypocrite, but at least I tried my hardest to ignore. And I'm not a fighter, unless provoked. I wasn't pregnant either. It wasn't the words I was trying my hardest to not hit, it was my little brother's actions (spitting, licking and hitting me on purpose).


My little sister... she can be... disgusting at times. She dug a pregnant test out of a dumpster and licked the part where you pee on it. She even licked her own dried period blood. She always eats most of our baby sister's (4) food and doesn't care.
She also sexualizes herself by twerking and taking mirror booty pictures.

I just hope this is a phase and not her personality.