I am giving another angle to your prediction...
here it goes...
okie when arnav asks her(rather coax her) to go ahead and open the report if she does not trusts him...
she feels ashamed for what she did not trusting him and landing up hurting him ...
she refuses to open the DNA report ... and asks for forgiveness ... Arnav offcourse will be angry and will insist she opens .. but khushi says otherwise... this continues and arnav leaves from there ... khushi feels horrible and runs out crying ...
all these watched by sheetal.. will pick up the dna report ... and smirk...
yes the dna report will be exchanged to a real one ... and will put into use when time comes...
till then she takes advantage of arnav being angry with khushi and grabs this opportunity to get close to him... well atleast to an extent where khushi notices the closeness ... so when khushi notices this she decides on opening the report .. which would have been already replaced with a fake one ...
she opens it to find the test positive!!
waddya this?