I think Nakul should still go ahead and tell the truth to tusli dadi and the SN folks. Now that he knows that Mayank is his real brother he might have a heavy heart, but in the interest of TRUTH (which is what his Tusli Dadi is all about!!) he still tell the truth and provide closure to a whole lot of people.
To do justice to one brother and his fiance (and not to mention to do justice to Eklavya and to Manthan also) he must sacrifice the love for his other brother.. he should not get bogged down by real vs. not real brother.
After all, ek hi din mein jo bhai jaan se pyaara tha (lakshya) woh bhai paraaya kaise ho sakta hai?
I think its just a matter of a few episodes before Nakul tells the truth out. (But now he also has to live with the fact that the fairy-tale mom Ganga is not his real mom, but Dino Mom is his real mom! That is a real tragedy for him!)
If Nakul tells the truth, then Manthan-Nakul enmity will also end!! 👏
👏👏