WIBTAH for not telling my sister that I slept with her ex-boyfriend?

I (17 F) fucked up. For some context, last November my sister's (23) on and off boyfriend (24) invited me to his house. (My sister lives in another state and the guy and I live in the same city, different state.) When my sister started dating this guy about 2 years ago everything was wonderful. Its like I got a bestfriend. He introduced me to various things gaming, photography and so on. I genuinely admired this guy for how cool he was. My sister was happy for our bond. Last October, his mother invited me to their for lunch. I agreed because I had no plans for that day. When I reached the house, he greeted me at the door and said that something important came up for his mother at work and she had to leave in short notice and invited me in for lunch. I didn't think twice because we done this before. After lunch he called me into his bedroom to show me a new project that he was working on. He explained a bunch of things and I could see that he was contemplating about something in his head. I asked him what's up and he kind of suggested this. "My mom's not home and you're in my bedroom. So you wanna do something?" And fucking leaned in for a kiss. The thing is, I did not flinch nor did i refuse, I fucking went along with it. Soon enough one thing lead to the other and we had sex. My mind was blank and I was screaming at myself for letting him do this and though of my sister. They were broken up during this. I felt sick to my stomach. I went back to my dorm and I couldn't face any of my roommates. Maybe a month later I told my close friend about it and she freaked out. She stated the obvious that it was a grown ass man and im still a minor so it was illegal and to file a case against him. The thing is, I dont want to. My sister got diagnosed with severe depression around that time, so things were really hard for my family and I didn't want to add more to the problem. Its not like I wont tell my sister about this. I will. But im waiting for things to calm down at home. Last month my sister and him got back together and they've been calling. He kept on asking her on why I wont pick up his calls anymore, so she asked me about it. I told her that I didnt want to talk to him. She kept on asking me why and had he done anything wrong. I didnt have the heart to tell her. So I called him up and told him to stop asking about me to my sister. He pretended not to understand what im talking about and when I finally told him that im doing this because of the interaction we had, he said, "oh well, its not like i raped you're anything. You consented. Don't you dare go tell this around to people. Blah blah blah." I fucking blocked him So reddit, wibtah if I do not tell my sister about this now. Should I listen to my friend? What do I do? Aitah for sleeping with him? (I know that the last one is a stupid question.)