I'm afraid your brother in law is taking advantage of you and your wife on a huge scale. My instinct is to say that you and your wife should emigrate and leave them all to it.
Your predictions for the future are undoubtedly accurate - you will never be free of your brother in law if you continue to allow him to use you in this way.
He is a free-loader who will be a burden to you and your wife for the rest of his living days. He has got away with it so far, so why should anything change? He is living the life of Riley, getting money and groceries from you, getting his rent paid, getting you to do the caring and work.
When your baby is born you will both need to focus on caring for your new son or daughter - not supporting a lazy freeloader who will never do anything for himself, or for you.
You deserve better.
Run for the hills and take your wife and child with you.