Yes, as they now know they share the same paternal DNA.

You're the boyfriend, aren't you?

Ancestry.com is probably eharmony for you right?

What the hell ! Of course it's an issue.

Can you explain to me why it's an issue? Aside from it being frowned upon in society which is neglible. And increased odds for a child being born with malfunctions, what is the exact problem in dating a family member?

It’s icky, that’s what’s wrong with it.

To be fair, incest was a societal norm amongst those of high influence for thousands of years. Mixing all of their good "blood" with the commoners would dilute it. (another side note) There was also a practice of having an older relative introduce sexual practices to younger family members when they came of age. Just because we look down on this now doesn't inherently make it wrong or icky. We just know now that it causes genetic issues and should be mitigated.

I’m a firm believer that consenting adults can do whatever they want. If a consenting adult brother and sister want to have a sexual relationship, I’m not going to be the one to stop them. I will, however, be the one audibly gagging at the thought of it.

I'm with you on this. Like you said, they probably shouldn't reproduce. But I think that its the social aspect that makes taboo into taboo. If a brother and sister are raised together, then one day they make out I would probably call them sick. However, in this situation the attraction came first. Yeah, the situation requires a serious talk, but they weren't thinking about their siblinghood before romance arose.

He’s her brother!! That’s why it’s wrong. Date outside of your gene pool. I wouldn’t care if it’s a step sibling, but it’s not, it’s her biological brother. He’s not adopted into the family or related by marriage.

It's actually called the Westermarck effect. If OP and her half brother were raised together, she would not find him attractive. In fact, if OP was raised with any guy, she wouldn't find him attractive.

The OP never said the boyfriend looks like their dad...

Multiple times I've heard the saying "girls marry men like their fathers". This doesn't only refer to the looks but also to the character of a man. We are all shaped by our childhood experience. If this woman adores and loves her father she surely will look for the same qualities in her boyfriend.

I’ve also heard that “boys marry their mothers.” These are understandable truisms. It just must be disturbing to know WHY the boyfriend resembles their dad so much.

you realize people move to small towns too