Good Morning Paul,
May I ask why your former stepfather is staying in your home? Is it your grandparent's home? What is the relationship?
I know your thinking is bothering you and you are trying hard to accept the change but it feels unfair to you as you are the one that has to give up something due to the actions of your former stepfather.
Accepting change is hard. Try to take it one step at a time, first accept that you do not have control over this change but don't let it throw all the good you have been doing into the dump.
Make the upstairs bathroom yours, decorate it a bit with your own personal style. Remember to stay in the moment. You cannot go back to yesterday and tommorow is not here yet so live in the moment.
Gentle Hugs
Kitt