My sister seems to be drifting from her deen - need advice

Assalamu alaikum, I’m 19 and my sister is 21. We’re from India and she has basically been like a second mother to me growing up. She taught me how to pray and always reminded me to hold on to our values. Lately, though, I have this worrying feeling that she’s drifting away from her religion. A few years ago she stopped wearing the burqa and then even the hijab went away after she went to college and stayed in a hostel. Dad tried to advise her to cover, but she said many of her friends don’t and that it’s common in the city. Most of her friends are non-Muslim and there’s maybe one other Muslim who also doesn’t cover. In the past months her clothing has become more revealing - skirts, shorts, tight dresses, and she goes to the gym in sports bras. Dad is embarrassed to speak to her about it. Mum and I have tried to talk, and she replies that this is how people dress in big cities and that it’s normal. I worry about the influence of her friend circle. Recently she posted a story on social media showing her and friends at a party-like place with drinks and cigarettes visible on the table. That caused a huge fight at home. Dad warned her that if she doesn’t come back to the family way, he will consider her choices very seriously. She insists she doesn’t drink and that she still prays daily and is religious. When I asked her privately she admitted to smoking but denied drinking. She’s blocked most of us online, so we don’t know what she’s really doing, though she still calls home every day. We don’t know if she has a husband or boyfriend - she does have many male friends who aren’t Muslim, which also worries us. I’m confused and don’t know the best way to help her. Dad, who is usually calm, is very upset. Mum and I don’t want to push her away but we’re scared for her. Any advice on how to approach this with wisdom and compassion would be appreciated. JazakAllah khair.