Well she's one of the nicest warmest people you'll ever meet for starters. I was definitely attracted to her from the moment I met her but because she was with my brother I tried to push those feelings out of my head and I think my subconscious came up with a loophole of sorts by creating fantasies of her being nurturing to me that weren't overtly sexual. Stuff like helping me get dressed in the morning, bathing me then reading me bedtime stories etc.
Even though I furiously jacked off to these scenarios I could justify it to myself since we were never sexual with each other in these fantasies. Growing up there wasn't really any porn available so I had to take what I could get, the only source of female nudity in the house was a book on pregnancy and I had developed a fixation on a photo of a spread eagled woman screaming joyously with a baby starting to crown, I probably jerked off 200 times to that picture before we finally got decent internet and I started downloading ideepthroat clips and whatnot, even when I had moved on to "normal" porn that picture still dominated my subconscious and it was only natural to picture my brothers fiancee screaming with the same orgasmic joy.
She is an avid softball player and her team has a facebook page so it wasn't too hard to find celebratory pics of her screaming with joy, exerting herself as she swings the bat etc. that would do perfectly, I'm a bit of a shut in and the only pictures of me are lazy half smiles after my mom yells "come on, look happy!" during christmas holiday gatherings etc. so I pinched myself in the arm till I my eyes watered and I drew blood and then took a bunch of snapshots of myself fake sobbing with my computers until that point unused webcam. I then downloaded a pdf of the pregnancy book and photoshopped my face and body emerging from her and then downloaded a solo preggo porn gallery of a woman with a similar body type and photoshopped her head and myself gradually crowning.
By this point I was kind of done fooling myself there was "nothing sexual" going on and ended up photoshopping my head and hers onto a gallery of the same pregnant woman getting fucked but after that moment I had kind of a "what the fuck is wrong with me" moment and guiltily deleted that gallery but not the birth one but still pretty much decided this was getting out of hand and knew I had to move on. I hadn't jerked off to that gallery in nearly 4 months when she asked to use my computer and had almost forgotten about it when I realized I needed to do something because literally anything was better than stumbling upon that.