There is no way Nivren be separated...
Look at Nivi, she is so much enjoying being with Viren, she never even repents for being married to viren..and she never even thinks of Sid other than frnd or her Jeeju...
so there is no question of Nivrath reunion..if nivi had still thought of sid as her boyfrnd or lover then there would be chances of viren sacrificing, but we dont c anything as such..and i dont think viren is such immature character that he is jealous of nivi concentrating on Dwitha..