Many such cases are there. Parents not willing. But the Daughter/ Son never want to come out of their decision. Marriage is not for a day or two. Once a boy and a girl marry, they should be together life long. But many of these love marriages are not that long-lasting. Today they love, tomorrow they marry and the day after tomorrow they divorce.
So as a brother, you have a greater responsibility. The interest of the boy and girl are more important than the interest of the family. Before going for any further action, your first responsibility is the know completely about the boy. What type of boy he is, how is the financial position, how are his parents, what is their family background. You for any adverse points?
Then ask your sister that she can't have both egg and chick. She has to select one. The boy or her family. Tell them that the parents even may not allow her to their house after marriage. This you should tell her when there are no adverse remarks about the boy. If she still says she will marry that boy only. Let her proceed.
If there are any adverse remarks about the boy, you clearly discuss with her and see that she will forget him, if the adverse remarks are very serious. You should convince her that if she marries her there will be many problems. She may hear or may not. Then leave it to her.
Now make an attempt to convince your parts. Try to talk to them separately and if one comes out and agrees, it will become easy for you to convince the second. If you try to convince both together you may not be successful. Talk to the mother and make her understand and then go with her to your father and both can try to convince your father. If they agree OK. Otherwise just forget and see what is going happen.