Inappropriate infatuation with partners relative

I'm a mum of three, I'm in a relationship with my partner of six years and we have had lots of fights and a few break ups during this time but have always managed to make things work and when I think of our children it makes me not want to give up on our relationship.
Over the past 6 months or so, I have developed feelings for someone who my partner is related to. This is an awful feeling but I have dreams all the time about him and some are sexual, which is quite awkward. I have butterflies in my tummy when I think about him even though I don't want to have feelings for this person and I try and snap myself out of it, I try and think of how it would ruin my relationship if I ever acted on it and it would ruin my partners family. How do I get over this? I feel awful