Well actually there are diff opnions about this situations every character has their own pov
As much as i remember during the coffee date meghna told kunal that one of the reasons she didn't accepted him was that her mother wouldn't have approved it so from this we can say that sharda s values doesnt allows her to accept these things most probably that s why she was shocked to know meghnal s consummation its just the way one thinks
Naina didn't knew about the convo that sharda and meghna had all she knew that her di who never ever had even a bit of fun took a big step so most probably she loves and trusts her finance completely that why naina took such a drastic step to protect her sisters s happiness plus she saw her mother for whom her swabhiman is everything going on knees and begging hence the big decision
Its very easy to label there thinking as backward or old fashioned but i think one should try and understamd there pov too sharda solanki is somebody who fought with the world and raised her girls of meghna and naina are today known as iit and iim toppers then a lot of credot goes to her mother s thinking and her guts
One cannot call her weak she obviously did biggest stupidity of her life when che choose a boy for her daughter just based on some informations