There are many things coming into play here:
1. The hostility towards the Mongas: Even if they want to trust Sahiba, Seerat and Santu’s deceit throws them back to square 1.
2. The dire circumstances of the wedding where both bride and groom are at loggerheads: The respect of both families was left in tatters which both Angad and Sahiba had to pick up.
3. Sub-human treatment of Sahiba by the majority of the Brars: The taunts, denial of food, lack of human decency.
4. Sahiba not knowing the rules of the game…i.e how to behave with whom, where to show cunningness, and understanding what can backfire on her - Like that first day if instead of Jasleen, it had been Gurleen, then this siyapa wouldn’t have happened. Gurleen could have spoken with her husband Jaspal who could have called the police in front of Sahiba and let her speak. This is also a test for brides going in the sasuraal. To be attentive and not trust anyone. Granted she had been in a terrible state of mind, we can ignore this incident. I can bet you, Sahiba will be much more vigilant and will triumph just like Sansa did in Game of Thrones!