I have no idea what to do in this situation so would love some other people's perspectives.
Apologies for the gender-non-specificity here but don't want to be too outing.
One of my siblings has a partner who I will call 'Alex'. They have been together for a few years and live together.
None of us like Alex very much. By 'us' I mean my other siblings and our dad. We thought Alex seemed pretty awesome at first but over time have come to all agree that they are really self-absorbed, controlling and do not seem to make our sibling very happy. This view is based largely on what we've seen in terms of their interactions but is also based on what sibling tells us directly themselves.
Sibling frequently complains about how awful Alex is being; how Alex wants to get married but they couldn't possibly make that commitment to them given their shortcomings; how Alex's demands are unreasonable and making their own life hell.
Here's the tricky part: despite all of this complaining, sibling also says they love Alex and don't want to break up with them now (although might have to do so in the future). Sibling goes from bitter complaining to expecting us all to play happy families - lots of time together over Christmas; treating Alex as part of the family; frequent invites to theirs and expectation of reciprocity. This means that we all find ourselves having to pretend we don't know things we know and act as though we like and respect Alex when we don't.
To make it harder still, Alex is pretty isolated - no family in this country; few friends - and has told me and one of my other siblings that we are basically the only people they trust and like spending time with. This makes social situations with them all the more excruciating from my perspective, because as much as I'm not Alex's fan, they are still a human being with some good qualities and it feels awful and cruel that I'm being two-faced with them all the time. Alex has even picked up on dislike from other family members and tried to talk to me about this issue and how to solve it. I'm not a good actor / liar and don't believe it's right to treat someone in this way. Worst of all is knowing that Alex wants to get married and that my own sibling has no intention of marrying them but having to listen to Alex talking about their hopes for the future. I'm basically colluding in Alex being strung along.
I don't know what to do. If I distance myself from Alex I will be distancing myself from sibling too, isolating them and pushing them closer to Alex when they present as needing my support. If I'm honest with Alex about my worries about their relationship with my sibling or about not wanting to be matey, I will definitely cause offence and my sibling will be angry and upset with me. If I keep faking it around Alex I am being cruel to them and untrue to myself and a pretty despicable human being.
Help!
Just to add to avoid drip-feeding: this isn't a situation where I think my sibling is being abused by Alex or fear for their safety... or at least I don't think so. With controlling-type behaviour it can be hard to tell.
YABU - keep your mouth shut and keep being friendly towards Alex. Maybe it's not ideal but it's the best choice.
YANBU - you should do something else here - distance yourself from Alex / find a way to be honest with them / insert better idea here.