"In college with my big sister" sounds like one of those Japanese comedy anime that are not entirely family friendly unless you're Japanese or Scandinavian. Luckily this is not anime but The Sims 4, so what could possibly go wrong when putting two co-dependent siblings in a dorm room together?
I guess that depends on what you mean by "go wrong"...
"I don't really have any reason to sit here but my sister is nearby so my life is complete for now."
"Homework is boring, but I can endure it as long as my sister is here with me."
"What? My sister thinks it is super funny when I do that!"
And now for MY rants!
"Why are all these non-sister people flocking to me?"
I said it is my turn! This is my big hate about Uni, it takes the worst feature of the base game: Multitasking socializing, and dials it up to eleven and a half. One, two or even three dormies will be drawn to you if you try to do something productive, and slow whatever you do to a crawl. Even if your social bar is full, as it mostly is in a dorm. Even if you are in your room, unless you lock the door. And them not being family, I can't just make them go do something useful.
Solution: Make them do homework right after classes, not in the dorms. (Going to the library gives a speed boost to homework, but there will be people socializing again.)
Rant 2: When you go to the dorms, your family is instantly transformed into standard friends. They will randomly appear in the dorm without warning, but mostly they will call you asking if you want to hang out, or go clubbing with them, or go to a festival with them, or my favorite: Challenge you to a magic duel in the arena. Never just call and hear how you're doing. And you never get to go home and spend a weekend or something. They're just old friends now.
Not bad, mostly good: The food! My sims can both conjure food, but they never have to, as dormies come home with family portions of various foods, from cookies to Lobster Thermidor. (In my test sims' dorm, they only brought cakes, and one of my sims became malnourished. It is a thing now.) One of the contributors is Erika here, or "dat ass" as I called her the first sim days. (The pants came with the university expansion, and yes your bum looks big in those. Putting them on is like gaining ten pounds instantly. OK, maybe eight.)
Not bad, just weird: One of the dormies felt the urge to spray with disinfectant, including the food. Don't try this at home, kids! That said, after she stopped, Mass and at least one of the dormies got sick with fever and dizzines.
Mostly good in moderation: A couple of the dormies have high violin skill and will randomly start playing in the dorm. Watching them raises fun for everyone, so they perform a useful service. On the other hand once you have two or more of them performing at once, it is time to take the headphones off.
All in all, this expansion badly needs some way to tune down the automatic socializing, and it would probably not give me the "tense" and "uncomfortable" moodlets. As it is, however, it does. But I endure it for your sake, noble reader. You deserve to know the truth!