This isn't really etiquette-related, although some smaller etiquette points will play into it, especially if I decline.
BF and I have been together for a long time. We're definitely headed towards the latter end of a decade. Anyway, his parents and I get along fine and I like them a lot. So... his mom has invited me along to a female-family-members-only weekend. It's going to be her, BF's sister, and a handful of relatives on his mom's side (BF's aunt, cousins, cousins-in-law).
Even though his mom and I get along very well and I adore her (I'm grateful that she's so awesome, given some of the horror stories I've heard and read about), I'm pretty hesitant about accepting this invitation. So, here goes another of thebadchemist's lists:
-I have never spent time with BF's family without BF.
-Due to the fact that his extended family is scattered all over the US and they don't get together much, I have met very few of them. Of the people in attendance this particular weekend, I would only know his mom and his sister.
-I will be the youngest by 2 years and beyond that, at least 15.
-I will also be the only unrelated person there, as everyone else is related by blood or marriage.
-I feel bad because every time I spend time with BF's family, they pay for everything. I don't want to be another mouth to feed and another person to buy a ticket for, etc.
Obviously, the pros to this situation would be the potential for bonding, fun, etc. I also have no idea how BF's mom would feel if I said no. BF said he wouldn't be offended at all if I declined, but that's largely because he knows in a reciprocal situation, he wouldn't go. He gets along fine with my parents, but due to cultural differences and a language barrier, he probably would feel pretty awkward on a weekend outing alone with either of them.
Anyway, I'm not really sure how to proceed, so any feedback would be appreciated. Thanks!