The fact that you know about this weekend and were told about it by one of your friends, actually shows that your feelings of exclusion are unjustified.
If they were excluding you, you would not have been told aabout this visit. It is likely of someone is feeling down, that a friend may well invite them round for a quiet weekend. I suspect that I would do the same in the same circumstances.
It probably never occurred to either of them that you would feel cut out, they would just think that someone dear is feeling down so you invite them round, not for a get together of girls, but as a hand reached out to a friend in distress. If you had been the one feeling down, not your friend, the same hand of friendship would probably have been offered to you.