What should i do in this situation?

So the thing is that i belong to a strict Pakistani family where it is considered ill of opposite genders interacting with each other. My father works abroad and i live here in a joint family with my uncles and i am kinda head of my family cuz my father is abroad.
I was suspicious of sister chatting with someone cuz whenever she was using the mobile and i went close to her so she would be like shocked and quickly changed the screen ( i didnt intended to spy on her ), i ignored that but i knew the fact that somethings was up but i did not wanted to accept this cuz i trusted her and believed that she would also trust me.
Two days earlier she came to me and told me that one of my uncles was checking her mobile phone and found her chats with a boy whom she was texting ( she wasnt in a relationship with her, just texting ) and my uncle being a typical Pakistani uncle told her “well see about that”. When she was telling me this whole incident she had tears in her eyes and i comforted her because personally i am not against texting ( between boys and girls ) but when this texting leads you to do something un-Islamic and something that would would lead to disrespect of our family and name of my father that is the thing i am strictly against of because personally i look at everything in my life from the eye of islam, so it is permissible to text unless it creates lust full desires in peoples heart. Then she said that it was nothing but simple texts, i believed her and told her not to worry and i’ll handle the situation of it gets out of hand and that your brother is here for you if u ever need him.
Then today i checked her mobile personally while she was in school ( she doesnt know thatvi have been through her personal things ) but to my surprise the things i found left me shocked. She was texting to like one or two boys and the text were all from her side.
The first boy whos name was ahmed, she tried to be in a relationship with her and even asked her that if he loves her, but he denied for some reason and no such relation was established but she told that boy that she had a crush on her cousin. HER FCKIN COUSIN. Now let me tell you something this cousin of hers which is also my cousin is my best friend like i trust him more than anyone in this world and i know that he wouldnt do such a thing under my nose but the thing that hurts me is that in one of her texts to the boy ahmed she told him that she had kissed her cousin ( my cousin and best friend), now i cant digest that and a part of me thinks that she did this text just to make Ahmed jealous because he rejected her and i dont think that my cousin would ever do such a thing to my sister but still she texted that. And she also made a story in which she combined her picture with my cousin ( it was just an edit, but still).
I think my mother was also suspicious of her due to the same changing screen habit and there is a high chance that my uncle told my mother about the mobile checking incident.
Now i dont know what do to with her after finding all these things. I am afraid that if i confront her she will be over secured and will start keeping further secrets from me ( she is still keeping secrets but she reached me when she had a problem ) and if i ignore this i fear that these things will grow and eventually get out of control.
I was thinking of telling my father about all this situation cuz he is kinda cool and he always has a solution for these kind of problems but he is abroad and i fear that he doesnt get depression thinking about his daughter doing these things her while he cannot do anything.
So can you guide me on my this situation i truly need some good advice.