Hi Reina,
What will probably happen is ONE DAY your middle sister will get herself in a big, grown-up situation where she simply cannot talk about it with the baby sister. YOU will be the one she runs to if a guy dumps her, gets her pregnant or is bad to her. YOU will be the one she runs to when she has trouble in a job, can’t hack the university, or has a health scare. Then you will bond by trauma, like soldiers do.
Also, you shouldn’t care what a high schooler or a university aged student thinks or is doing. They are simply not ready to join you in the grown up world yet. Let them have fun together. That easy time will change soon enough, as we know.
Best,
Inky