I go there with boyfriend, leave my knickers in a plastic bag because used and left on floor to put into suitcase next day to go back home ?
Do you have a husband and a boyfriend? Or are you calling your boyfriend's mother your "in-law"?
I have a concubine. When i met his mother, we lived together and i was introduced to her as " his concubine".
Therefore, since, it's always been understood on both sides that she was my mother in law.