The only reason I can think of is that Ektaji made contract to forbid her from revealing the truth to Bani! 😆
But on a serious note, you are right in saying that this whole mess can be made a lot easier to handle if Karuna revealed the truth... Although it is normal for stories to be stretched out over a long period of time, there is usually good reasoning as to why characters can't find out the truth easily. In this case, I personally cannot understand why Karuna isn't telling anyone! 😕
The only possible reason I can think of is that Karuna is in fact a negative character with an ulterior motive.
If that doesn't turn out to be the case, then Ektaji will need to come up with a watertight explaination behind Karuna's silence on the matter!
God bless.