Ammu, trust is like an eraser it gets smaller and smaller after every mistake!
Kanta feels she is dealing with someone who makes a habit of gradually winning trust & then betraying. Earlier Shaurya had won them over (especially during his stay at Sharma's residence) only to jilt Mehak at the altar, took her mother's recipe book, razed their shop etc, ,she has seen Mehak shattering & breaking into pieces,Mehak nearly lost her life ,as a mother she is understandably suspicious of Shaurya.
Kanta sees him as a negative influence on Mehak's life & feels Mehak is vulnerable to his charm and might go through the same turmoil again, hence is being overprotective. Shaurya needs to patiently win the Sharma's over & gradually convince them of his genuine feelings for Mehak 😳