I am 17 and I really like this girl I would do anything to be with her but every time I go to talk to her and try to say "she can do better then him" she defends him however her boyfriend is 6 years older then her and cheats on her regularly,I tried to ask her out when I first met her but she nows doesn't talk to me and blocks me (this was before I knew she had a boyfriend mind) but it makes it awkward because I know who he cheats on because my older is friends with the cheating boyfriends sister how do I show her that she can do better then him because every time I see her angel like face I fill bad that if I actual say anything unless she would reject me and I will never be able to help her but every time I see them she always goes touchy,feely type and when I look at them and they always she always tries to kiss him to show he's not that bad ,can you please help

I know you like this girl but hey, she has a boyfriend although, perhaps not a very good one but still she's taken.
She doesn't want to talk to you and has blocked you( on I'm guessing some social messaging site?). So, she obviously doesn't want to talk to you, or about her
Boyfriend. I guess in the end, she will have to learn the hard way, and that is her problem, not yours.
Keep bugging her and she will hate you even more. But, better yet,look for a girl that is available. So, you answered your question... YOU can't be with her.

Ps-- your story is a little bit hard to follow... proof read if you expect some answers

I don't know but maybe try to get some evidence that he's cheating on her and try too show her that he is bad for her or something.

You need to let her find out on her own that he is cheating on her. The last thing you want to do is get involved considering you two are not that close nor do you want to be the one to break the bad news, or she could take it the wrong way and think your making it up, because she already gets the hint that you like her. Just let time tell, and back off. And she's probably being touchy and kissie with him when your around not to show that he is a good guy, but maybe for you to get the hint to back off.

Just be her friend, and don't focus on this one girl. You might have a crush, but a crush is a crush.. and sometimes you can't change someone's heart to the same as yours.