Raghbir left his family's business to build his own empire; and the reason was Bani. What if, there was nobody in the family who was capable enough or diligent enough to run the family business. Could be the rest of the family are freeloaders who depend on Raghbir to be the breadwinner; and they lost their breadwinner when he decided to open his business. By killing Bani; they could get Raghbir back to work for the family; and they could easily control the financial matters of the family. The mother, especially, is very greedy and blinded by money and fame.