My Guess

I think Sanjana used to love Raman who was married to Shagun.But she technically know she will never get Raman until Shagun is out of Raman's life.So taking the chance of Shagun and Ashok's affire she tried to come close to Raman.

Then Shagun used Sanjana to create misunderstanding between Raman and Adi.By saying Raman is cheating Shagun and maintaining a relation with Sanjana.

Then in court also she might have given statement against Raman which led Raman to looses Adi and a chance for Shagun to file divorce against Raman.

This can explain why Adi hates Raman as he would be thinking that Raman cheated Shagun maintaining a relation with Sanjana and Ashok helped Shagun to settle her life.

Then after the deciet from his sister Mihir might have started hating his sister.May be Sanjana ,Shagun and Mihir are related by cousine relations.

Like either Shagun and Mihir are br and sis while sanjana is cousine sis. or vice versa.

Now sanjana is planning to get back Raman so to create void in their relation.

waiting for the mystery to finish

But Sanjana is mohra nothingelse. She is saying rattamaru dialogue. She is saying whatever shagun or param told her to said. In both function ishita called Mihir sister and shagun was present their without any invitation that means shagun is Mihir sister. Sanjana is doing for money. She didn't come to party because it was family party and only family member was there and her plan will be flop. She came to tilak because she knew many people will be present there and chance of getting caught are less. I think shravan and ruhi will hide jewellery and toshi Amma will investigate everyone and either Sanjana will caught or shagun will enter to divert everyone attention. Adi hates raman because shagun brainwash him saying that raman used to beat her and don't love her anymore and it's not shagun but raman who gave divorce and ashok help her in bad time

I guess very soon we will get to see who Mihir's real sister is.