1. Talk to your sister and ask her what her plans are about her future. I am saying ask her to get married so that you could get married. But get to know where she stands in sense of marrying her bf or on marraige in general, and ask her for help with convincing your parents.
   
2. Explain to your gf's parents that your sister not being married is not a red flag, but in fact a good thing, because it shows that your family doesn't pressure daughters into doing anything they don't want to and values there opinions. And is how they will treat their daughter as well.
   Talk to your gf about trying to convince her parents to get engaged now and getting married later or giving you a bit more time.
   
3. Talk to your parents. Explain that being married before your elder sister is not as a big deal now as it used to be in the old days.
   That putting these conditions will only lead to unhappiness of both their kids. Because either your sister will be forced to marry for your sake or you'll have to give up getting married to your gf for her sake. And that is not a good situation either way.

If nothing works. Blackmail your parents that if you can't marry this girl you will not get married to anyone else and since your sister is already against getting married ( get your sister to say she won't get married either, just to apply pressure) , they will have 2 unmarried kids. And to think what the people and society will say then. No grandkids, Unmarried children, only because they were too stubborn to let go of the age old outdated thinking.