Dear Student,
Mir Qasim, the son-in-law of his predecessor Mir Jafar, was made the Nawab of Bengal by the Britishers, even though they had also backed the appointment of Mir Jafar. This was because after the victory of the British in the Battle of Plassey, the newly appointed Nawab Mir Jafar made a treaty with the Dutch East India Company, behind the back of the Britishers. Dutch naval ships were also spotted in the River Hooghly. This then led to the battle of Chinsurah, where Mir Jafar allied with the Dutch. Hence, after their victory, the Britishers forced Mir Jafar to abdicate and replaced him with his son-in-law Mir Qasim.
Regards