If I went on a vacation with my life partner and they spent the whole time sleeping with their other partner and leaving me alone in a separate bed I might get passive aggressive too! Even without sex, falling asleep together can feel wonderful. If there a reason she didn't get to share his nights on the trip?
Do you know if she even wanted you along on the vacation, or was that all his idea? If it was something that was supposed to be a special get away for the two of them and suddenly he announces that you're coming too and now her special escape with him is a group adventure instead... again, I can see why she might be pissed.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.