GC application pending in family category by sibling, dependent visa G4 chances

[rsdevulapalli]

Hi, I am 40ys old and my older brother filed greecard application for me in 2014(Family of US citizen-siblings), case pending. My visitor visa got rejected twice since then showing no apparent reason. My fiancee is indian citizen working in US for UN on diplomatic work visa G4 with 3yr contract, extendable further. What are the chances of me getting G4 dependent visa after marriage. We wish to get marry soon and live in US while on the contract at UN. My fiancee's organization sponsors the dependent G4 visa after marriage. Are there any restrictions on issuing other visas while greencard application is pending. Your timely advice is greatly appreciated for me to take life changing decisions.
Thanks you
Responsible citizen of India