Happy Family?

I've  been sleeping with my boyfriends sons and daughter. He's 50 and I'm 30 and he has 2 sons aged 21 22 and his daughter is 24. I feel bad but I also don't, he's horrible to his kids and can be quite mean. It started when I accidentally walked in on his son masturbating and he just smiled and carried on so I watched then he asked if I wanted to give him a hand, I was hesitant at first but I did because it was just us and ended up sucking him dry and as a thank you he went down on me until I came. He didn't tell anyone the next time was about a week later when I thought I was home alone it was really hot so I decided to sunbathe naked for a bit then his other son appeared asked if he could join me and just led back beside me naked and I couldn't take my eyes off his cock it was the biggest I'd seen in person and he noticed. He asked if I wanted to touch it and I actually jumped at the opportunity and as I was holding it it actually got bigger because  he'd turned himself he had a full view of my pussy and said he could see I liked it and he was right we went upstairs incase anyone came home and we fucked it was slow at first because of how big he was but we got faster and he ended up coming all over my ass. I was feeling really guilty at this point but they aren't my kids so it's not that bad I kept telling myself. There wasn't any mention of it until that weekend when their dad went on a business trip. Then we were having lunch and his sons started whispering to each other they then walked over placed their cocks on my plate and asked if I wanted something more filling. I didn't think I just dropped my dress and kept switching between sucking one then the other. I had in my mind that this would be the last time so I'd go all in and we had an amazing threesome right there in the kitchen. But that night the guilt got to me and I ended up crying into a bottle of wine in the bedroom which I think his daughter heard. She came in and sat beside me and asked what was up, I confided in her and she said it wasn't my fault it was her dad's and we're all just trying to be happy. She kissed me on the cheek smiled then kissed me again and I just went with it, kissing touching fingering she started licking my pussy then my ass which I didn't realise feels really good. We ended up having sex a couple of times that night and the next day over breakfast we all ended up talking about the sex and their dad we all agreed we'd carry on but not get attached it was strictly just sex. He doesn't have a clue about any of it although he nearly caught me and his daughter once but a we just shouted not to come in because we were trying clothes on and she doesn't want him to see her naked.