@gymandgin
It is never nice to feel left out especially when it concerns a friendship group of your other half. I was not in the exact same situation but my DHs best friends wife seemed to have an issue with me, my DH or us both.
Over the course of some years, they would snub invitations to things or the husband would come on his own. While the situations (yours and mines) are quite different, the advice I can give you is that it is best to not let these sort of things get to you. I always used to get upset and anxious about this women not approving of me (she snubbed an invite to my hen) but over time I just did not care anymore. She did not see me as a friend and while I never really cared to be hers, I will admit I liked the idea of double dating and couples weekends etc, which you have had.
In my experience, things can change over time and people see the error of their ways. Only recently this same couple changed their tune towards me and was suprised when I did not go to their babies christening.
I think you might be worried about being excluded from wedding related things and feeling like your BF and his friend, fiance, sister etc are all part of something your not.
The wedding is one day and the hen one weekend, try to get through it as best as you can and afterwards, your BF will be focused on you and only you. When you get engaged, just do not invite her to yours. It could be that she does not see your relationship as serious? When my DHs friend got married, I was not invited to the wedding and then a few years later when we got married, his friend said to him 'If we knew, Oceanblueeyes would be your wife, we definitely would have included her in our wedding'
I know it is so annoying but organise something great with your BF for the weekend of the hen or with your friends. Buy an amazing houseofcb dress or something as nice for the wedding and go look amazing.
After the wedding you then can decide if you want to continue these couples things. Personally for the sake of your BF, I would put up with drinks or dinner but draw the line at any weekends away again.