It used to be I would have drawn the line at attending the celebration but would have included the family member in gatherings. Now I would draw the line and include the family member ONLY without the other person. Even then I would only include them rarely. I could invite the person to lunch by myself but I would no longer be party to a gathering where other people, especially kids, might be influenced to lower their guard against this evil. It is a disease the infects others. When society rejects it you can more easily show mercy. But when the pressure is to accept the sin, mercy means harm to others. In that case you have to choose protecting the weak. The wayward family member did it to themselves and could fix the problem by simply turning from their wicked ways.
And no it is not like every other sin. This is a sin the government is forcing people to accept and celebrate. Other sins can be ignored and the family member usually already knows/admits it is wrong, or at least respectfully declines to flaunt it.
I should add that I do not throw parties so the issue never comes up. I am saying what I am saying because of the lessons of life. I’ve seen this up close and we sinful humans easily weaken our values. Love and mercy need to have the objective goal of truth and purity, redemption and restoration. Otherwise those pretty terms are just cover for advancing, aiding and abetting evil. Yes we all sin. That is precisely why we must be so careful.
If a family member is behaving immorally in more traditional ways I would not draw the line mostly because the traditional habit is to be hush hush about it — to have enough shame in the behavior to not announce and flaunt it or to force others to call it good.