My fiancés daughter turned out to be my son's girlfriend. Now everything has fallen apart
*Sorry if this is long, just alot of explanation needed here.
I (51M) have been with my fiancé(50F) for about 2.5 years. I lost my wife, the mother of my son (20M), to cancer 7 years ago. My fiancé divorced her ex-husband 5 years ago. They have two children together: a son (23M) and a daughter (19F).
My son knew I was dating someone, but we hadn't introduced our families yet. I also knew he had been seeing someone for a while and was planning to bring her over soon. He'd told me her parents were divorced, she wasn't very close with her mom, but that he'd met her dad. He always spoke highly of him.
My son absolutely adores this girl. They're both students at Yale ,she's pre-law and he's pre-med. He often tells me how smart and driven she is. He also said her father is a neurosurgeon, which is the specialty my son hopes to pursue someday. From everything he'd told me, her dad had been very welcoming and supportive of their relationship.
A few days ago, my fiancé and I decided it was finally time for everyone to meet. She came over for dinner with her daughter.
The moment they walked through the door, everything fell apart.
Her daughter is my son's girlfriend.
No one had any idea beforehand. My son and his girlfriend were completely blindsided, as were my fiancé and I. Dinner ended almost immediately because everyone was in shock.
As everything unraveled, I learned something else that has made this situation even worse. My fiancé had always told me that her marriage ended because they were constantly fighting and eventually grew apart. During all of this, I found out that wasn't true. She had an affair, and her ex-husband divorced her because of it. I also learned that's a major reason why her daughter has never fully repaired her relationship with her.
Now I'm struggling with the fact that my fiancé lied to me about something so significant. If she had been honest from the beginning, I would have appreciated the honesty far more than finding out like this.
To make matters even stranger, she also lied about her ex-husband's race. She had told me he was white, but he's actually Black. Their children are obviously biracial, so I genuinely don't understand why she'd lie about something so easily disproven. It wouldn't have mattered to me in the slightest.
Now everything feels like a disaster.
My son is understandably upset. There's tension between him and me, even though neither of us did anything intentionally. There's also tension between my fiancé and me because of the lies.
If we go through with getting married, my son's girlfriend would legally become his stepsister. I know they're not biologically related and didn't grow up together, but the dynamic is incredibly uncomfortable for everyone involved.
I honestly don't know what to do from here.
Edit: I should probably add another important detail because people will likely ask about my fiancée's son.
The affair wasn't with a coworker or someone her own age. It was with the older brother (25 at the time) of one of her son's friends. It was completely consensual, but understandably it caused a lot of damage within their family.
That's also why her son didn't come to the dinner. I had assumed it was simply because he lives across the country (he's currently in medical school at Stanford) but I've since learned their relationship has been strained ever since the affair. The distance wasn't really the reason.