I think there are actually three people to blame for this one is Mili, second Abhi and last but not least Simi!
Abhi I think is responsible because he should have maintained distance which he didn't. He allowed her to kiss him and was also holding her hand and grabbing her taking her to places with him. Any girl would have gotten the wrong impression. He is responsible for this himself, I understand that he wanted to help Mili but he should have always made sure that the relationship that he has with Mili is of a brother in law so he should treat her as a sister in law not like anything else like a friends because you can always fall in love with your friend and there should always have been a boundary. Even if they were friends there was no need for him to hold her hand, drop her off to places. If you really think about Mili's interpretation isn't wrong.
Simi is responsible because she should have talked to Abhi about the closeness he has with Mili the day Mili kissed him on the cheek. Instead of going to Mili and telling her only she should have told Abhai to maintain a distance too. What was Mili going to do? Abhi was holding her hand and grabbing her along places, All in all Simi should have confronted Abhi as well and not only Mili.
Mili is responsible because come on that is your brother in law. How are you going to do something so disgusting to your sister who you love so much.
This is only what I think I don't mean to offend anyone.