I had to read this 5 times to fully comprehend, but thank you!
Okay. So. Philip is the product of "incest", and that bit of the argument only applies to Philip's father, really.
So, if the twins weren't messing around with one another, what was the Aunt on about? And where did the incest rumours between the twins begin? Didn't the maid say something about seeing the twins messing around?