I don't want to give my room to a guest
Why do you need to give a room to a guest?
I have to take care of my husband's brother who will visit us in a month.
It would be nice to give a room to your husband's brother. I am sure your husband will appreciate you helping out his brother.
I am going to make sure his brother has a bad time so he does not come back.
Do not mistreat his brother. Your husband may resent you for that, as you should be accommodating to guests in your house, especially family.