My sister and I are both like thIs we think it's hypocritical to break bread with anyone we don't like. When her son in law visits she really struggles as she cannot st,and him,
But she does? That's the point. If you think you can go through life avoiding everyone you don't like and never having to make concessions (like your sister does) it's going to pretty miserable and people will move away from you. I imagine if your sister refused to eat with her son in law, it would amuse problems between her and her Dd.
Op it's sounds like your problem isn't the being nice to someone's face, it's the bitching.
Quite honestly I have a relative (by marriage) that hates me. She has gone out of her way to be nasty to me and my kids. Really awful stuff. But when I see her I am civil. Mainly because it makes it easier for everyone else but also because it winds her up even more when I don't bite and it ends up with her looking more of a dickhead. Yes I am fake with her. But it doesn't benefit me at all not to be. But I don't talk about her behind her back.
And yes I do think people who are 'fake' get further. I try my best to be honest, but tactfully. At work they use me as the person who clears the shit in meetings. I am known for cutting through the waffle and getting to the heart of the matter. But manage to do it without pissing people off. Which involves some fakery 'I totally get what you are saying. I am here to support you and the team in achieving your goal' etc. What I want to say is 'cut the bullshit, what went wrong? Once I know that we can fix it'
I don't then come out of a meeting and say 'God Bob is such a dick'.
A tip for avoiding the bitching, is to politely remove yourself. It used to happen when I worked in a call centre. On breaks we would be having a cig and the team would slag someone off....as it started I always needed to make a call, or go to the loo, or desperately needed to speak to someone else who was out there about work. I didn't need to do any of the above, I just pretended. You can remove yourself from the conversation without being confrontational or judgmental.