ASR calls his Hone wale Sali / Bhabi "Aap" and his Hone wale Biwi "Tum"!
However... he calls "La" tum coz he has know her for a long time and they worked together / dated etc! So she wont be "Aap"! Too formal! La calls him "ASR-tum" as well! No formality there! (On a totally unrelated note: I HOPE ASR-LA, stay friends!)
He always called Khushi "Tum"! In the beginning it's coz he was just being a jerk to her, but NOW it's coz he knows her well and they are too familiar for him to be that formal with HER!