Is it cheating?

I've been dating this guy almost a year and we just started having sex. He's decent all around. My twin brother sets a bar high on how women to be treated and my bf doesn't exactly meet that. Last night I caught my twin brother jerking off. I went to go get my iPad out of his room and he was naked watching porn in his room. His penis was scary big. I just wanted my iPad, so I took it from him and then saw that he was watching porn about brothers having sex with their sisters. I asked him about it, he didn't deny that he had thought about. He actually admitted that when I wear certain clothing he leaves because of his erections. I was flattered. I don't know what drove me to do it, but I sat on the bed next to him and began stroking his massive member. It was so hard and thick. Seeing his response got me wet, and made my pissy swell and throb. I couldn't hold back, I got on him, even though it was painful, I took all of him. He stretched me so wide, I thought he was going to turn me inside out. We fucked so hard, long and passionately. Best orgasm I've ever had. I've definitely got to do that again. His cock really shows me the whore that I am I love it.