on mother in law demise flat would be transferred by society in name of the 3 legal heirs . nominee is only trustee for legal heirs . your BIL is not the owner of the house
2) your brother in law can in respect of his one third share nominate whom soever he so desires
3)you can add your son as nominee in respect of your one third share of flat
4) your BIL cannot object as you are free to nominate whom so ever you please