I have taken to many classes in psychology. I recall studying about this phenomenon. That is a very normal reaction. IT occurs often with your bf's father or brothers, and the opposite for the guys. Your mom or sisters.

You obviously have an attraction to your bf.There is likely some resemblance between the two either by means of looks or by means of mannerisms. So if you find one attractive then it makes perfect sense to find his dad or brothers attractive. If you have sex with your bf, then it is normal to think about your partners father in a sexual way. It is just something that happens.

I had similar feelings towards my H's brothers and father in the early days of our marriage. My H also shared his guilt over having an "interest" in my mom and my sister. I reassured him it was perfectly normal, and it is.

Good luck with this. I hope it passes quickly.