This happened just over three years ago when I was a senior in high school.
My mom had been dating a rich guy named Steven for six months and we just moved into his house before Christmas when they got engaged. I don't really like him, but my mom LOVES him and he basically took us from a small apartment in a bad area to a huge house in a good neighborhood. He is handsome and has money so I'm happy for her, but he was saying sexual things to me which creeped me out and kinda turned me on at the same time. In front of her, he would call us his "beautiful girls" and make comments about what I was wearing and what about me I got from my mom. He said stuff like "I'm glad she has a boyfriend so there aren't all these strange boys knocking on my door…"
When she wasn't around, Steven complimented me more sexually and one time he hugged me and his hand went down my lower back to my ass. The most obvious sexual thing, and the catalyst for our sexual encounter, was he walked in the kitchen in just his boxer briefs when I was eating breakfast and my mom was in the shower.