Did I just witnessed my sister being raped?

I was staying a night at my big sister's house and me and my little 14 year old sister were watching tv. My sister was upstairs with her boyfriend and she said they were going to watch tv up there. I went up there to ask her if I can cook some ramen noodles. I opened her bedroom door and I saw her in lingerie with her wrists tied to her headboard and duct tape was over her mouth. I asked what was going on and her boyfriend said they were just playing around and told me to go away. My sister didn't seem scared or upset or anything and she just giggled and looked at me. After about 15 minutes they came out and they were holding each other and laughing. I wonder if I just witnessed my own sister being raped. Did I just witnessed her being raped or where they just playing around ?