I think she is just pretending to be good. & thus from past two years she has made a goody goody image in front of Arjun's mom. & this time she wanna make her place in Arjun's heart.
& this Rano is overly good in front of everyone & such overly good people may damn dangerous for everyone. (Remember, how Nisha was on those initial days )
PS: Rano's will be vamp & it's confirm by Sara herself, so it's sure she is pretending to be good😆