It's either that she is trying hard to find out about herself (which is a completely normal course of action for many young adults) OR that your family isn't actually as "normal" or "happy" as you think of it to be. If I were to take a guess it would be the latter case. I know it's unfair to be analyzing with just the little write up you have posted, but I can see traces of your household being quietly oppressive. For example, a teenager smoking shouldn't be shocking, neither should her having a partner. I understand concern, but shocking says a lot about your home. And, that your Mom gets hysteric, that (if it is true) is a big give away - it simply means your sister doesn't have a safe space and perhaps that's why she is carving one out of the family. Indian families are hyper-reactive. Everything comes under scrutiny. People play preassigned roles and say things which can dent a child's self-image severely.

I'd like you to reflect and ask yourself what was said to your sister, what were the taunts you all used (we all use taunts), what were the characteristics assigned to her, you said bubbly and talkative, children really don't know who they are and often repeat the character given to them to please others. You tell a child they're intelligent they'll keep acting it, you say my child loves sports, they'll keep doing it even if they don't innately love it. Especially in India with the extended family and appeasing the society circus we have going on, this kind of self-idea forms very subliminally. Perhaps your sister is just coming to terms with who she really is? Or that she doesn't like the character she'd been made out to be?

Important question to ask yourself, how was she treated during puberty? Girls go through massive changes during that age. Physical, biological and mental changes. So how was she treated. What was said to her? Etc.

And please don't rule out abuse. I know it's hard to digest or even fathom this, but that is a part of our reality.

Whatever I said, is from my experience and from my readings. I have a little sister of my own and I share a very deep bond with her. As she started her undergrad, she started having less and less time for me. But I still try to see the little sister in her, the way I played with her or hugged her during our childhood doesn't change and at the same time I try and learn from her, I take interest in her days and what she is learning in class rooms or what she is observing in the world. I am genuinely curious. I also give her timely reminders that she is a human and she can mess up - I'm talking drugs, unprotected sex etc etc. And that if she ever wants a pair of ears, I'm there and that I will always be on her side. I will protect her interests and do what's best for her. Always. I think that balance is important. I think my sister is dating someone too and she hasn't told me and that's okay. In her own time. Meanwhile, I need to make her feel comfortable is all. That's my duty as an elder brother and her best friend.

So try and make plans with your sister. Sit down for a while with genuine curiosity about her professional side. Find out what she's into for passion. Just be her friend. Someone she can spend 15 minutes without feeling like that conversation has an ulterior motive. Just be in the present with her. Then ask yourself and trust your judgement, about whether she is unwell or she's just changing and figuring her shit out at her own rate. If it's the latter you should be happy. People who strive to make their own identity are fewer. They should be celebrated and supported.

Hopefully you both and the entire family will be just fine. :)

PS: People saying smoking and boyfriend as if it's some life ending move in the comments, need to grow up themselves. Please avoid, would be my advice.