While Sonali and Diandra are obvious choices to have got escaped the eliminations, Minissha and Soni were clearly in the danger zone, thanks to their invisibility powers. Soni and Minissha were playing the game way to safe which put them out of the limelight, which is potentially the biggest reason why contestants get eliminated from the show.
Soni's biggest mistake was to get too comfortable in her group and get too far away from the rest of the household. It is a known fact on the show that groups don't survive long and friendships don't last. Though Upen Patel and Soni Singh's reign went on as long as they had a big family comprising of Karishma Tanna, Diandra Soares, Arya Babbar, Sushant Digvikar, they lost their chance when the group started disintegrating.
While surviving in the Bigg Boss house is difficult as a sole contestant, moving forward on the show in pairs, be it as a couple or a brother and sister, is twice as difficult. Since Upen is the friend of all, Soni lost her chance in this fight from the beginning.