Chowdary Girl and Reddy Boy marriage - convince father

Hi.. I wanna marry a responsible person who belongs to Reddy family and their family is welcoming me always and no issue from their side. They are not forcing me for anything.

I have been informed to my first elder sister one and half year ago and she didn't even agree to let my parents know. Finally ended up with informing my parents myself about this relationship. My parents were angry and they didn't talk to me for some days.
Now.. My mom is ok with me but my father is not with me like earlier and he told to my sis and brother-in-law that he wont talk to me if I marry him. Issue is with sub caste.

before this i want to let you know that i took up the responsible at home and did everything for my sister's marriage.

Now brother-in-law is saying that he did BG verification about that guy and noone told that he is not good. My sister and he is ready to do a marriage with the same guy but my mother and father won't be present for that. I told to him that i can't go for marriage without my parents.

How can I convince my father in this matter. Please help