I slept with my advisor's daughter and she is blackmailing me now. What can I do? I am doing a PhD in the US. I am in my mid-twenties and will graduate next spring. My advisor was very good and nice to me. He invited me to have dinner at his home many times and then I met his daughter. I did not realize at the beginning but she told me she had a crush on me. I have known her for some years now. She is above the age of consent.

I went to a party last week and had an extra couple of beers and I could not resist her. I am here alone and had not been with a woman in two years until then. 

Now she wants to start a relationship with me and is threatening to tell her father, who is my PhD advisor, that I seduced her if I do not agree. 

This is a very difficult situation for me because I do not want my advisor to think I took advantage of his daughter and the trust he has in me.

Also, I do not have time for a relationship. I just want to finish writing my thesis and then graduate. 

Aside from those issues, I would love to date that girl: she is cute and hot. In normal circumstances, for me it is extremely difficult to get a girl. I am like Leonard from the Big Bang Theory TV show.

I do not know what to do. I just want to graduate and go home. But she is threatening me.

What should I do?  What should I tell my advisor? I do not want him to feel betrayed and that I took advantage of his daughter. She is the one that came to me because under normal circumstances I cannot get a girl. But also I do not want her reputation to be damaged. 

I tried explaining that to her but she told me I got sex and I want to get away with it. I am afraid and worried.

Some additional information: There is no way I could have seduced her. I am not good with women. 

I do not know what happened. As I can remember, I had very few interactions with her. Never shared anything but a few encounters in social meetings. That is all. And then she tells me she has feelings for me. I did nothing to get into that situation. I was invited to a party and then she appeared. I had a couple of extra beers and she offered to take me home and there she jumped over me and kissed me and I could not resist. I had been a very long time without sex. 

I am very worried and scared.
First off, you haven’t committed a crime (assuming she was above the age of consent) and you haven’t engaged in academic misconduct (since there is only an indirect academic relationship between you). However, the fact that she is threatening/blackmailing you is very concerning. The fact that she is your advisor’s daughter makes things more complicated, but being threatened or blackmailed by a sexual partner would be deeply concerning in any circumstance.

I do not know the exact nature of the threats or blackmail, but I would not engage in a relationship with someone who is threatening or blackmailing me. Only you know the exact situation, but your choice of words rings alarm bells. My primary concern in your situation would be worrying about whether she will make worse threats — e.g., false rape accusations. That would be much worse than a spat with your advisor.

If I were in your situation, I would try to record my conversations with her so that I have some evidence to protect myself if she does carry out her threats. Save text messages and get a voice recorder app on your phone. (Check the laws on recording conversations in your state first.) The only thing other than “he said, she said” you have to defend yourself is such evidence.

Maybe I'm misreading the situation from your question, but your choice of words "threatening" and "blackmail" is concerning. And there is no harm in trying to protect yourself from possible future accusations. Hope for the best, plan for the worst.

This is not a complete answer, but I think you should worry about being threatened/blackmailed before you worry about dealing with your advisor.

In terms of dealing with your advisor, two (conflicting) points: Generally, honesty is the best policy (as Buffy's answer emphasizes), but she is an adult and her father has no automatic "right" to know about her sex life, and there is a good chance both of you would prefer him not to know (as Nicole's answer suggests).

In terms of dealing with her: It sounds like you are still on speaking terms with her and you should absolutely try to talk things through with her. Treat her with respect and acknowledge her feelings. (See Geoffrey’s answer.)