Re: Is it ok to joke like this ....?
^JazakAllah khair brother for the kind advice, but I think it would be very difficult as Turks seem to find it very odd to be called sister by anyone except her own biological brother.
About the colleagues, they would find it "Highly" offensive and my past experience says that they would ignore my such comments.
One more thing I noticed recently, the comments have become more indirect than before. Like when he came to my room, he saw anything which he thought might be related to her, he said, " Hmmmm.....Chori pakri gayi"
or something like " Aaaj barey udas ho, lagta hai kisi ki yaad a rahi hai" ( The day when the girl is absent)
You can't even say anything on such comments as the person might later say that he was talking about something else.