What should I do about my grown unmarried brother-in-law entering my bedroom without permission and sleeping there when I’m not home, instead of using the guest room? My husband believes it is Wajib for us to accommodate him, but I find this unacceptable.
Bismihi ta'āla
This is definitely inappropriate, and something should have been done from the very first time.You are right, it is unacceptable, and very wrong. Your husband must stop him, and if he does not, then you must stop him, or involve your family or elders to stop this, or move out with your husband to a place where you have your privacy.
And Allah knows best