Feel Guilty

Then talk to your sister. And apologize, for you should have been paying more attention to her and her new fellow than to this other guy.

Now, I am not saying that you need to pay attention every second. And a party is a place for lots and lots of people. There are distractions. But I think she has the right to be a bit upset with you, and the buddy's wife has the right to be upset as she undoubtedly thought that you and your new pal were going to have sex upstairs in her house. Never mind what really happened, or that you would never do it, or whatever - the wife did not know any of that at the time. Her leap of logic is more like a small hop.

So - you're sorry. And offer to take your sister and her new fellow out for a quiet dinner somewhere - your treat. Explain that you want to get to know the new fellow and want to do so without the distraction of a party. It will be just the three of you.

As for the wife, you can give her a small hostess gift if you like, but not out of guilt. Maybe just a pretty plate or even a card. This is somewhat optional these days (it used to be something people did a lot more of; this is a practice that is dying out). Don't mention the incident in the card. Just say that you thank her for opening up her beautiful home to you and hope that you can see each other again at some future happy occasion. Mail the gift (if it's a plate or something breakable, pad it well, of course), and be done with it.

And remember for next time, okay?

Ye gads, what WERE you thinking? (not only using the home of a host you didn't even know, but for going off with a guy you don't even know)

Were you drinking?

You acted inappropriately. Don't send her a gift. It will come over as being condescending.

Apologize to your sister and ask her to apologize to the hosts. Most likely they have an opinion of you that can't be smoothed over with a little gift.

I hope you learned from this experience. Most people choke on humble pie.