I'm just saying that I love my sister enough that I wouldn't want to jepordize our relationship over something stupid like inviting someone over for Christmas.
I love my sister, but I love my extended family as well. If this were so simple as inviting someone over for Christmas, what I'd probably do is have an additional small Christmas gathering of people (preferably younger) that I'd include my sister's new GF in on.
However, if we're talking extended family members whom I
ALSO love dearly, with older people who have deeply rooted beliefs in the sanctity of marriage as well as deeply rooted beliefs (right or wrong) regarding the homosexual revolution, I'd have to consider their comfort levels and my relationship with them as well.
I'm not going to be placed in a position where someone I love demands that I take sides. If my sister, whom I love dearly, gave me an ultimatum that I had to invite her to a traditional family gathering with her new lesbian partner otherwise I didn't love her and wasn't supporting her, she'd get standard lecture #49 about how just because I wasn't doing what she wants doesn't mean I don't love her. She can deal with
that as she chooses. Ditto for older family members.
It seems that the OP's sister isn't demanding that, though. It appears that she's "taking the temperature" of her family's feelings by asking her sister how she'd feel if she brought her new lesbian GF to their traditional family gathering. The sister has offered an "out" for her sibling: I can make other plans if you don't think it's a good idea.
It is my opinion that the sister hosting should take the "out" and tell her sister to have a wonderful trip with her new lover. There will be other Christmas gatherings. This isn't the be-all, end-all gathering and it will keep as much peace as possible in the extended family.
That's what I would do. What the OP does will be whatever is best for her family. She knows them better than I do.
**Edited to Add**
Ok, I've gone back and read ONLY the OP's posts. As usual on the DIS, things got blown way out of proportion and I was mixing in other people's posts when I was evaluating my stand on this.
It seems that the gathering we're speaking of was small to begin with: OP, her mother and her sister's family, which included her BIL of 21 years. That gathering has been irrevocably changed due to the new dynamic of the sister leaving her husband and starting a relationship with a woman about a month ago (September?). The sister is now demanding that she bring her new lover to the small traditional gathering on Christmas eve or she's not coming.
OP doesn't seem to have any issues with the lesbian thing, only that the dynamic of these three people and their tradition has changed and she feels under pressure. The nephew will feel the change as well, even if the lover doesn't participate, because it used to be just him, his parents, his aunt and his grandmother. Now it would be him, his mother, aunt and grandmother.
I'd have to say that the tradition is now dead and it's time for a new tradition.
I also stand by my previous opinion about people who give ultimatums and insist to me I don't love them because I won't do what they want me to do. I refuse to be coerced in that manner and won't be controlled by other people who insist that I have to prove my love for them. It didn't work when I was a teenager in the back seat of a car and it won't work now.