Doubt

i m nt sure if d curse part is actually mentioned by vyasa...but it is a popular beleif...brc shwd it...

It is actually there in KMG translation. .I shall post citations ...currently thru mob...

One more thing yudi also says after he found out that karna was his brother that he had always wondered why karna's feet resembled kunti's feet for him...

He even does water rights for karna along with karna's family...

She was very much concerned about her son Karna dear. Just before the war she went to Karna, revealed the truth to him, and asked him to join their side when Karna said no. That was out of desperation. But it was too late. Karna was also always with Duryodhan, and always sided with wrong knowing that what he is doing is wrong. And finally it was war time and if then Kunti would reveal the truth to everybody including Pandavs, then they would definitely not fight the war and the gainer would be Duryodhan.  Why would Kunti let that happen? She lived all her sons but she also wanted Yudhishtir to get the throna was the was the rightful heir. So she tried, and revealed the truth to Karna. But it did not work.  

Kunti never really cared for Karna, she only went to Karna to protect her sons, Kunti states this as her reason for visiting Karna.

It was Krishna who offered Karna everything, the former offered him the throne also, Karna knew that Yudisthira would give him the throne if he ever knows the truth, hence Karna requested to keep his secret a secret.