In a co-operative housing society in Maharashtra the person whose name stands first in the share certificate is known as member and others as associate members. According to the latest amendment to the Act an associate member has also to be a beneficial owner. If your mother-in-law's name also is to be added a new conveyance deed will have to be executed. It will involve stamp duty. In the event of death of one of the three, the other two can continue one as member and the other as associate member. The legal heirs of the deceased can have claim on the notional share of the deceased, which they can claim by contesting if necessary.
Your son can be nominated even now. A guardian will have to be appointed to stake claim during his minority. A nominee cannot become owner of the property. He will only hold it in trust until transfer to legal heirs.
If you want to add the name of your mother-in-law and do not want her legal heirs to claim rights, you will have to give appropriate reason for adding her name. For instance you can mention in the deed that she has not contributed anything towards consideration, nor is it a gift in her favour. You can say that her name is added only to give her a right to live in the house in the unfortunate event of her son predeceasing her.