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Hello and welcome! My name is ***** ***** I will try my level best to help with your situation or get you to someone who can.
If you are living with boyfriend in his father's house, then legally the father is your landlord and you would be considered a month to month tenant under an oral tenancy agreement. But the same as any tenant, you have certain rights to privacy and the right to exclusive use and possession of whatever area that you agreed that you would live in.
So the boyfriend, who is not even your landlord, has no legal right to enter your room or prevent you from installing a doorknob with a lock on it to prevent access. You would have to give the father a key because as the landlord he has to have the ability to access the area in the event of an emergency. But neither of them can simply barge into your private living space or touch any of your personal property.
If one or both of them did so, they are violating your rights as a tenant and your right to privacy and you could file suit against one or both of them for violating your rights if you can prove who is entering your room. In most situations like this, a tenant will get a "nanny cam" which is a hidden video camera which is motion activated to catch the person in the act so they can be held liable in a civil suit in small claims court.
As an aside, in addition to being an attorney, I have also been a landlord for over 26 years...