Should a Syed boy marry a Khan girl according to Islam?

Before some time i met a muslim girl and now we like each other.But she is khan and i m syed boy.Last month her family did the engagement with her cousin then i told her to obey her parents decision. But she dont like him at all.So i told her to talk with their parents. But before some time she told me that her parents has died when she was 1 years old only and now she is living with their uncle. Her grandmother is still alive so i tell her to talk with her.Still i dont met her only see her photo & talk on phone. I know that i have done a gunah because Islam not permitting it but now i m in tension because she has no real parents and if she choose any wrong way that is more big gunah for me. So i want your help please tell me what i have to do now; 1.Is engagement authentic according to islam? 2.Should we or she talk with her grandmother and family for this? 3.Is islam allow a girl to choose a boy or not? 4. A syed can marry with khan girl. “I ALSO REQUEST YOU FOR PRAY FOR BETTER”