Your practical angle in a way seems logical. Yes, Malti and Vinu need to be fixed permanently. Actually, all the injustice done to Sadhna was indirectly because of Vinu-Malti. If they were matured and repsonsible, then this situation wouldnt have arrived for her. You are right, instead of blaming the one taking advantage, one must act wise and not fall into the prey.But in all these Sharmaji has been blamed and tortured by Vasu time and again. In the form of house and by ill-treating Sadhna, Vasu has troubled Sharmaji a lot. And the credit goes to Vinu-Malti.
The reason why Sharmaji is getting the grunt is because he is the head of the family and the house technically belongs to him. Due to Vinu-Malti's acts, it is Sharmaji who is fallng prey over and over again... Vinu-Malti are holding the gun on his shoulder and firing. Jis ki kandhe par bandook hai, usse pe tho vaar kiya jaayegana??? Sharmaji is letting all this happen by not fixing the cause...