since your in laws own the house,they have the final say.They probably liked the idea of the extra money the tenant was giving them..well who wldnt.
I guess unless you agree to take over the tenants rental n pay tht same amt to your in laws then it wld seem fair.
Moreover you r not married to the family yet so your opinion probably doesnt matter to the family.
Just get a place to rent n tht wld be the end of your problem.The rest you can slowly figure it out.