[Vicky.]

Well since this ones not so hypothetical for me (though I added a few bits)..this ones easier to answer

I would/did tell my father.

Spoiler:

As it turned out, he was not as fragile as I first thought he was, and had been wondering why she was acting strangely. They split up. He never told my mum that it came from me..so I never fell out with my mother either. Though I did feel anger towards her for quite some time for putting me in that situation to begin with.

Happy-ish ending though..they got back together like 2 years ago or something. And there hasnt been any word at all of any infidelity this time round. And they seem much happier together after the time apart


Spoilered this bit so as not to influence peoples answers at how 'well' it turned out in the end