I randomly started watching this show and then got hooked to it thanks to the cute banters.
Now the latest promo the slap by Maltiji seems like a disgusted reaction from her for a proposal as below:
Sudhaji does not want a "divorcee" for her son. So maybe to teach them why "a divorcee" seems unfit for her son she might either ask for Appu's hand for Mohit (who is also a divorcee) OR ask for Aaru's hand for Rakshit as she still isn't married.
Something like this might make her so angry to slap her. Just a wild guess. 
