I was that woman for a while. It was a very hard decision to let them have sex without me. But I did. And it was hard. Lots of panic attacks. I found out that the only way I was ever going to be okay with it was to let them do it.
That doesn't help you, but I just wanted to share that. I think you should have a conversation between all three of you before you go away. If you have the mentality "it's better to ask forgiveness than permission" in these cases, you could royally mess up everything.