Girlfriend’s Sister

This started over 2 weeks ago. My girlfriend lives at home with her parents and I like to stay the night there occasionally. She works extremely early in the mornings at a coffee shop while she’s in school so this is the only time we get together with my job as well. About 2 Sundays ago, I stayed the night and she left and I went back to sleep really tired. When I woke up I turned the tv on in their living room. Her little sister walked out of her room that connects directly to the living room when she heard me. She asked what I was up to. I told her I was just going to hang out for a couple hours and go to work myself. She said that if I was bored I could “hang out with her in her room,” and then walked back into her room. I wasn’t 100% sure what she meant by this but I chose not to think into it and sat on my phone. She is really pretty so I just tried really hard not to think about it, that was until I heard her say my name. When I looked up she was fully nude in her doorway. She just smiled and walked back into her room leaving the door open. I thought this was just crazy and went in. I told her that it wasn’t right and she shouldn’t be doing that. She confessed for the last 6 months I’ve been around, she’s been laying down hints I never noticed or picked up on. At first I tried talking to her but admittedly I was too weak. Within minutes of walking in, I had her pinned face down into her bed balls deep in her. Her mom works from home so we had to be quiet but it was really hard. It was by far the hottest and most pleasurable sex I have ever had. Felt so good I couldn’t bring myself to pull out so I had to leave early to get a Plan B for her. I felt so miserable after but like all bad temptations, I couldn’t resist again the next Sunday. Except that morning, as soon as my girlfriend left, I snuck into her room again and she laid there naked all night waiting for me. What followed was 2 hours of absolute pleasure. Now I sit here trying to decide if I should leave my girlfriend for her and ruin everything for something that won’t last at all, or to confess and hope we can find a way to move forward or end it between us. Either way, if we wait until this Sunday leading into Monday, I know I won’t resist and we will fuck again. Wish I could say confessing helped but I still feel like garbage