Attracted to my dad's girlfriend, is this wrong? how can i get the thoughts out of my head?

back in july, my dad's gf had a baby. my dad is 53 and she just turned 28. she has big , nice , nice body and his part hawaiian or something like that, so her skin color isnt white or w/e its called! A few weeks ago i was thinking about how great it would be to see her feeding the new baby and i must be lucky, because i happened to go down into their basement and see her feeding. she didnt see me and then i went back upstairs. i only saw one of her **** out and it was frickin huge! i would love to spend the night with her. shes not my step mom, technicially, but could be down the road. are these thoughts wrong? when i was there that same day, my dad was worried that he had another kidney stone because he had some blood in his pee again! gross. he's battled kidney stones off and on for years. he ped one last october and just ped one early in august. i heard that its like having a baby for a man...i imagine having a pebble come out of your can't be fun! anyways, back to my thoughts...how do i get her out of my head? yes i know there are other girls my age to look at and there is one girl that im kind of dating, but something about a 28 year old like her has me going crazy!