I think Twinkle is right. She knows Yuvraj well and so does Anita. Both of them suspect that Yuvi is up to something. He has the following advantages if he's pretending:
1. Kunj keeps his mouth shut out of guilt and allows him to do whatever he wants.
2. He can forcefully stay with his "dost" all the time and keep an eye on Kunj, even harming him, but not be blamed.
3. He can easily ruin Kunj-Twinkle's relationship by coming between them.
4. By not telling Anita the truth, he maintains that she has a hatred for both Kunj and Twinkle, not just Twinkle alone.
I just hope Twinkle doesn't start believing his acting.