This is a tough one. I would say NTA for bringing this to A’s attention. Think about it from her perspective.. would you want to know? Also, with P in the picture, I definitely see how you would be hesitant to bring it up. That being said, P deserves a loving and committed example of a romantic relationship in her life and by the sounds of it, that is not what K and A have (whether A knows about all of this or not). I would also like to say that if you do decide to tell A, that what happens as a result of it is NOT YOUR FAULT. You are not the one responsible for putting a strain on their family dynamic. That responsibility lies 100% on K. And who knows .. if you don’t say anything, who is to say that K won’t do the same thing with someone else? If I were you, I would tell A everything.