Madrigal is first mentioned as a cousin of Thomas in blood rites:
"I don't know," he said. "Guess I can do what my cousin Madrigal does: find some rich girl." He grimaced. "I don't know what to do."
So somebody else fathered the twins Madrigal and Madeline.
Exactly, but they belong to a different House, don't they? I mean while they are all White Court Vamps, there are differences. The House of Lord Raith feeds off of emotions related to sex, the House that Madrigal and Madeline belong to if I remember correctly feeds off of fear. While the term "cousin" means they are related, but to what degree? Close, as in first or second, or is it more distant as in still related but it is several generations removed. So was Madrigal and Madeline's father or mother a sibling of Lord Raith? Thus making them a close relation to Thomas or first cousin.. Or does it go back further as in perhaps a great great grandfather of Lord Raith, they'd still be related to Thomas, could loosely be called cousins, but not all that related. It could account in the differences in what their demon feeds upon.. However it tells us very little about fertility and their breeding habits..
The information Thomas gives us in the chapter is 1] he made an assumption that he was most likely
sterile. Why would he make that assumption? Logical to assume because most of his House or even White Court Vamps in general are.. However because obviously his father was fertile, he and Justine apparently used birth control.. 2] That Justine has a 50/50 chance of survival because the demon feeds off of the mother during pregnancy.. So half the the mothers die on average, do their babies and their demons die along with them? If they do, that would severely limit the number of members in the House.