I went into a deep dive into how likely it is Cersei and Jaime's offspring would have birth defects
On the plus side, they don't have worse odds than any other pair of siblings that aren't twins (yay for them!)
On the other side, the baseline for sibling offspring is children have a 20 - 40% chance of having birth defects when their parents are siblings (not so yay for them)
Math: If we use a mid-range risk of 30% per child, the chance that a child is born healthy is 70% (0.70).
Three Children: The probability of all three children being born without a defect is (0.70 \times 0.70 \times 0.70 = 34.3%).
The Actual Risk: This means there is a 65.7% chance that at least one of the three children will have a birth defect, which is far from 100%.
Why Past Results Don't Protect Future Children:
The outcome of the first pregnancy has absolutely no effect on the second or third. Genetics has no "memory." If the first child is born completely healthy, the risk for the second child remains exactly 20% to 40%.
This means that even after Joffrey was born "healthy", they still had the same probabilities of diseases for the next children
Conclusion: Twins shouldn't have sex. Lol