@Hoggirl sounds like you had a conversation about the trip with your SIL. Did she mention you joining? I am sorry they exclude you. I really wonder if they think you have no interest in participating in these trips. I understand feeling left out and have definitely felt that from certain people. (My MIL and SILs did/do not travel together at all!). However, I agree with those who say that if you want to participate, you may have to ask. Something like “Now that my life is less busy (or something), I would love to join you on a trip for a weekend or so.” Or just the three of you do a weekend somewhere. They may honestly think you don’t want to come and if not and they do not invite you, at least you would know. It certainly sounds like the travel buddies are not all on the same page.
I was with friends the other night and had a bit of FOMO as a subset had gone to Europe recently. However, when I heard how they structured the trip and what they did not see in Paris, realized that I might have been frustrated traveling with them and would have really have enjoyed the trip as much as I would with other friends.