I assume this is the post that you were referring to when you sent me a private message the other day.
What your sister is doing is her own business. If she is over the age of consent, it means that she is considered as an adult in your society; therefore, she has a free choice about how she chooses to live her life, and what she does with it. Your disapproval is noted here, but you have no right to interfere with her life.
What do you think you were doing going through her private messages and pictures on her mobile phone without asking her first? How would you feel if she did the same thing to you, and read all your private messages, and look to any pictures you might have on your phone you would be really angry to know that she would go behind your back and spy on you, so what makes you think you have the right to do that to her? What you have done there is despicable. You have violated her privacy, and betrayed her trust. You might dress it up as having good intentions, but there is no justification or excuse for reading other people's personal information without first asking their permission.
You are busy playing the concerned brother, but I suspect that this is more because you do not want to be associated with her behaviour. You feel it reflects badly on you, and therefore you want us to approve of you taking the phony moral high ground. Let's face it, if we were talking about your brother instead of your sister, there is no doubt that the vast majority of people would be congratulating him on being able to have sex with as many girls as he could get. But because it is your sister, she is being condemned as a slu
If you say anything to her, she will realise that you have been talking behind her back, and will probably figure out very quickly that you have been spying on her. She will feel betrayed and angry, as well as very hurt. What do you think that that will do for your relationship with her? At the very least, she will know that she cannot trust you, and she will know what you think of her, and I don't think that the two of you would remain sharing a place together for any longer than it would take her to throw you out. If she threw out, how would you explain that to your family? Would you betray her even further by telling your parents how she chooses to live her life? And for doing exactly the same as any guy would do, you included!!!
I only hope that she is insisting on her sex partners wearing condoms, to protect her from pregnancy and STI's. Beyond that, if this is how she chooses to behave, that is her right.
Something you need to consider very carefully as well, once a girl gets a bad reputation, everybody claims to have had sex with her, and then vilifies her in the next sentence. Your sister may have had sex with her cousins, she may have even had sex with two guys at the same time, but I don't doubt that half the people claiming to have had sex with her are doing so purely through wishful thinking, and envy of those who have actually done it. I would also consider very carefully whether someone who can talk about your sister in such unpleasant terms could really be considered to be a friend of yours. No true friend would say things that would deliberately cause trouble or hurt your feelings.
If you love your sister and care about her, then protect her. Don't allow your friends to badmouth her behind her back, and try to treat her and her privacy with the respect she would hope you should be showing. All of us do things from time to time, which other people might think are wrong, so before you act like man in a glasshouse with a pocket full of stones, think about the harm you could cause.