Machan,
you can marry .No issue. I dont think that you have a problem here..
http://en.wikipedia.org/wiki/File:Cousin_tree_%28with_genetic_kinship%29.png
Anyway im having something similar..she is my mothers sisters daughter. we went to a doctor to discuss this matter.She draws our family trees and came down with percentages.. and she said there are tests available in sri lanka but it costs around 2.5 lakh.from that we can identify whether there are any disease to be activated due to the marriage.call the human genetics unit in colombo. tell your story .They will give you an appointment.u can meet the doctor with your girl and get the answers( Privacy is saved).Its all for free.1 hour wage yawi..
http://www.hgucolombo.org/?page_id=221
So keep calm. Build your and her self,careers together.await for the permission( try to show some examples of similar marriages to ur parents. )
if the family still doesnt allow.... WAIT for the moment. If you stayed still..they will allow for your wish.its all about patience..
Wish you all the best and hope u will marry soon.Let us know as well. I too suffered alot due to this issue and now im fully ready to face it after the consultation of the HGU doctor.
Wish you all the best bro...Anyway you cannot get a suitable answer from a public audience.Most of them are telling what they have heard.But if you can meet that doctor, she will give you a clear briefing.forget the bullshits... Anyway you are having a blood relationship but its too far for a trouble. Just search and identify how many couples are there who are marrying the first / second cousins.. You r not the first one... Hope my post really boost you for a straight decision.
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