It must have been an awkward situation for the husband her; one moment he is chilling with his mistress, probably doing some romantic things to her. Then the next moment, his inlaws swoop into the restaurant and catch him red handed, cheating on their sister.
The inlaws felt so jealous on behalf of their sister that they decided to donate slaps to the adulterous couple.
That was way too far
Well! Maybe their sister is the breadwinner of the family and they felt the husband is flirting with another woman with their sister’s money.