Sharing Our Innermost Thoughts

My sister has a boyfriend. She is just 14 years old and he is 16. She had a crush on this boy who is also the son of my father’s colleague. She met him in their house where there was some occasion and his mother had invited our family. She got impressed by how he was able to communicate with everyone. He is a very good speaker ngl.
At that time he had a gf. After a while my sister was following him and her best friend who studies in his tuition told him about her feelings. And almost around that time he was very flirty. I checked their chats. It was already building up while he also had relation with that other girl of his school.
Now for almost 9-10 months they are in relationship. They don’t go out to meet often as there’s restrictions in our house regarding these.
So they talk online. And their chats!!! My goòdness. This boy is always like my so na my baby and my sis is like my hubby. They talk about physical things(not too offensive but there’s 2 meaning stuffs). Often day I saw this boy texted her about what she’s wear-ing. And tells her that he will give her this inner waer during their live in. like w×t×f? And today I saw them talking about kissing when they’ll meet. What should I do?😭 I feel so succumbed. She doesn’t know that I can read her chats. But she knows that I know abt her relationship. And sometimes I advice her but there’s no change…what should I do as an elder sis