My[35M] ex [35F] is dating my sister, how do I gothrough the holidays?

Hello to all of you, I have never done this so please tell me if I do things wrong.
Whe,I was 20 (in 2002) I met Nicole. Nicole and I hit it off immediately, we dated all through university, and moved in together. 5 years after we first started dating, we got married.
Nicole always got along with my family, my parents are very loving people and my older brother John and older sister Anne were always very kind to her. My youngest sister Jeanne was not a big fan of Nicole, but she is4 years younger than me and was a bit of a rebel and a troublemaker. Despite that Jeanne and I always got along great.
When Jeanne was 18 she came out as a lesbian. Our parents had a hard time dealing with it and I talked a lot to them, to not be so touch and to remember that she was still their child. I had many talks to Jeanne about her feelings etc.
Jeanne developed herself, got involved in gay groups, started to dress herself very stereotypical, again, I talked to my parents. They turned around, all was great.
A few months after our marriage, Nicole became distant, would often go out with 'friends', had to work 'late'. 6 months into our marriage I confronted her: Was she unhappy? Was she seeing someone else?
She denied it. 9 months after our marriage she sat me down and told me that she had been seeing someone else and that she preferred that person over me. That I was amazing and that I would make someone very happy, but not her.
It hurt, a lot, I tried to change her mind, tried to do everything but no. In the end I told her that I only wanted her to be happy, she was my true love after all, and that if someone else made her happy. So be it.
Nicole quit off all contact with me afterwards. Only to sign divorce papers she showed up. About 1 month later I got a call from my older brother John. He basically told me how sorry he was he did not call sooner, but he thought I needed some space. And how he could not believe Jeanne did this to me.
To make a long story short: Jeanne and Nicole started an affair. After the divorce they made it public to John and they told him 'I knew'.
I talked to Jeanne once, and I asked her why, and how she could do this? She replied that she was finally happy and that I should be happy for her.
This has been 7 years ago. Jeanne and Nicole moved away to a different country. They are still together. My family was really mad at them at first but the last 2 years my parents have accepted it, and John and Ann try to.
For 7 years I have not seen them. I avoided going to my family when they visited. I lived abroad the past 2 years so never saw them.
This year we are al invited. We are all going. I am not looking forward to it at all. WHat should I do?

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tl;dr: Ex and sister are dating, how to handle?


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