I (28M) had to travel overseas for a couple weeks for work related reasons. My sister's (19F) college is nearby, and she offered to feed my cats and water my plants while I was gone. I said sure thanks, and told her she was welcome to stay there while I was gone, since she lives in a dorm.
Anyway the day I got home, my sister was there with her boyfriend. That was fine, he's always seemed like a cool guy, but one thing I noticed was that my sheets were still in my washer. I didn't think much of it, I just assumed my sister waited to wash them, since she was using my bed while she was there. But she tells me, sorry, my boyfriend and I were "using" the bed last night.
That grossed me out. I don't think I'm being unreasonable to not want people screwing in my bed. My sister told me to relax, and said I was being a prude American (her boyfriend is from Ireland). She said that we (meaning Americans) stigmatize sex, and that she washed the sheets so it's fine. I said no it's not and told her to get out. She isn't welcome there anymore. Our mother has been reaming me via text, saying Imm overreacting.