I've had to deal with this as well.
My SIL was baptised and raised Catholic. She's on her 3rd non-Catholic "marriage."
My H uncle was baptised and raised Catholic. He's been married (Protestant weddings) and divorced twice and is living with another woman (the one he left his 2nd wife for).
My uncle was baptised and raised Eastern Orthodox. He's been married and divorced twice. First marriage civil ceremony. Second marriage Orthodox service. He has a girlfriend now.
A friend of my H's and potential future business partner, is Catholic and was married in the church 10 or 15 years ago (I don't know exactly when). He got a job far away from his and his wife's home state. They moved. Wife didn't like being away from friends and family. They separated. Eventually they obtained a civil divorce. Man was "single" for a number of years. He met another woman, he proposed marriage. She is also Catholic. However, they couldn't marry in the Church due to the fact that he was still married in the eyes of God. He looked into an annulment, decided it would take too long, and married his new bride in a civil ceremony at a country club.
Having fun yet? This doesn't include all the non Catholics married and divorced multiple times in multiple settings.
This has caused me no end of stress because of not wanting to act like everything was just hunky dory .. But I realized like the other posters that being polite and kind is not the same thing as showing approval for these awful circumstances.
Having them to dinner is one thing. Multply married and divorced Catholics sleeping under our roof ... that's an issue .. thankfully, everyone knows we practice our faith and it doesn't come up.
What can you do? You have to stick up for your principles, and if /when you have children you need to do the right thing as an example. How are you going to explain how Uncle John stayed over 2 years ago with his wife Kathy, and now he's here with his next wife, Debby ... ?
These folks make these decisions and it's their free will to do so, God help them. But then they shove it in our faces and we're the bad guys for not jumping up and down and holding a party for their latest romantic liasion.