How to convince parents to allow me to marry my sister's elder brother-in-law?

Hi,As I understand, you are in a relationship with a relative of yours who is elder to you and your family knows about it. And you have tried to convince them but have not succeeded yet.I assure you you are not the first one going through such a situation. There are many who go through such phase. Is age the only concern of your parents? If yes, give them time to get adjusted to this fact. If you and your boyfriend are serious about it and tell them that you are not going to marry anyone else, then eventually they will come around. But in case that is not the only reason, you need to give reasons for each of their concerns, and a assurance that you both will be able to handle those things.Both of you talking together with your families and assuring them will help more than you alone trying to angrily convince them. But for this, both of you need to be sure and be on the same page before talking to your families.Don't loose hope. I hope this helps.All the best.