I have similar experience as you.
My parents in law live in my husband's flat for 6-7 years. They don't want to move out even when we plan to get marry.......... unless my husband to buy him another bigger and new flat !!!! And my 99 said that I should pay her 'rent' and 'living expenses' if I live in 'her' flat after get marry. However, my husband pay all the mortgage and tax, water, electric, gas, mang. bill .......
Therefore, we decided to buy another flat when we get marry. This time, I told my husband I will pay the mortgage of our flat (my flat), and you only need to pay all other living expenses (include the credit card I used)........
And now, my 99 does not have my key.... And she should get my primit when she want to visit us.........it is because it is my flat, not his son's flat.
You are really very nice to them for the past 7 years. Now, I think you need to take it serious to let them know, if they are not moving out, your hubby will sell the house.