I agree that there is something here about her that is not right, and probably Shivanya does not know about it.
The fact that the panditji sends Shesha to help Shivanya itself makes me curious about her role. She cannot just be a sidekick in this show as she has put herself in harm's way time and again for Shivanya to enter the family.
She shows more hatred towards the family than Shivanya, which does not make sense. She hates the entire vansh of the family, which makes me wonder if she has a past with them. As she is much older than Shivanya - probably 100 years older - she may share a past with the family.
The poster is the biggest of them all. She also commented on Ritik's poster in the haveli, that he will face his doom. That is weird because Shivanya definitely does not think Ritik is a killer in this mess. I am of the opinion also that I think Ritik had a lookalike in the past - probably a king - who had some history with the naagin. Maybe it was a pichli janam thing?
About the poster, both Anky and Yamini have shown fears about it. Maybe they are aware of the intricacies of their son's past.
I also agree with the point by Fairytale, that Shesha impersonated Yamini and called him maali kaka. This is definitely some secret that nobody else seems to know about. Through this scene, they revealed that 1) Anky was a poor man and 2) somehow Shesha knows about it.
I think the CVs have really roped us in for a ride - first they revealed that it is Shivanya who is going for revenge. But I think during climax they may show that Shesha herself has some path for revenge that probably includes Ritik in the centre.