Its about my sister and her new boyfriend?

So, I have a neighbour and she is 16 years old. She is like my younger sister. She's always attracted to people who show her a little bit of care but she doesn't know if the care is true or fake. She had a long-distance boyfriend earlier and he dumped her. (He did not give any reasons).
I always advise her to date a guy who she knows but she never listens to me at all. From the past month, she said that a guy was seeing her but she doesn't like it. The guy has a coconut shop on a busy road. We both go for a drive in the evening and she always makes me drive twice on the same road where that guy is and takes me to a bakery which is located near his shop. I thought, she's just seeing him... nothing serious.
Then her mom did not let her go out with me for almost 3 weeks and I don't know the reason.
Today, she called me and said that she wants to come with me coz I go on a drive every evening. This time, she borrowed my phone and opened her insta account... and also she said that the coconut shop guy texted her and confessed that he loves her but she blocked him. When she was looking at her insta, I was getting continuous messages from my friends... so I grabbed my phone from her and I opened my telegram and I was using it but suddenly I wanted to see her insta account... so I opened it and I saw that guy's chats with my sister. I was shocked, coz the chats go on forever... and it clearly says that she's desperately fallen for him.
(That guy's age is 23 and my sister is 16) how can they fall in love in a month? She doesn't even know him. I too have no idea about him. I got a couple of screenshots of their convo... what should I do with it? What should I do?
Its about my sister and her new boyfriend?
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