FLOWERS IN THE ATTIC QUESTION!?

Ok, this is actually a question about petals on the wind, and it's really frustrating me and I can't move on until my question is answered. Nearer the end of the book we learn cathy meets Bart at a cafe near the woods and he asks how he is related to cathy. To which she says 'henrietta beech was your wifes half sister, malcomb foxworth had an affair with her and the outcome was three children, one of which was me' Obviously she was lieing, but I don't get how this would work because if henrietta beech was corinnes half sister, then how can malcomb have children with her considering she would be one of his own? Please answer because I really don't understand. Thanks!