Talk: Maegor I Targaryen
It's very misleading to call Maegor and Aenys half-brothers. When one parent is shared and the unshared parents are siblings, then the progeny are three-quarter brothers, not half-brothers. But because all three individuals are siblings, their offspring are actually more related to each other than normal siblings ever are.
Here's the proof. For a more rigorous explanation, see Types of Collateral Relationships
Mathematically, let R denote the coefficient of relationship, meaning the proportion of genes that are held in common by two individuals.
* R = 1/1 identical twins * R = 1/2 parents & children; siblings * R = 1/4 half-siblings; grandparents & grandchildren; aunt/uncle & niece/nephew; double 1st cousins * R = 1/8 1st cousins; great-grandparents & great-grandchildren; great-aunt/uncle & great-niece/nephew; half-uncle/aunt & half-niece/nephew * R = 1/16 1st cousins 1x removed; double 2nd cousins; great2-grandparens/children; great2-uncle/aunt; half-great-uncle/aunt & half-great-uncle/aunt * R = 1/32 2nd cousins; 1st cousins 2x removed; great3grandparents/children; etc.
With reinforced relationships, you must add all those up that apply.
So pretend for the moment that Aegon was not related to either his two wives, nor that Visenya and Rhaenys were in any way related to each other. If so, the value of R between Maegor and Aenys as half-brothers would be 1/4.
Now consider what happens when we leave Aegon unrelated to his wives, but Visenya and Rhaenys as (full) siblings. Now R for Maegor and Aenys would be 1/4 for being half-brothers plus another 1/8 for also being each other’s first cousins, for a total of 3/8. They would be each others' three-quarter brothers, not just half-brothers.
However, it is more complicated than that, because Aegon, Visenya, and Rhaenys are themselves siblings. That means that you must add in that Maegor and Aenys are again each other's 1st cousins doubled because the common grandparents are reached both ways. So when you add 1/4 to 3/8, you get R = 5/8. That makes them even more related to each than normal brothers are. That makes sense: Maegor and Aenys only had one pair of grandparents shared between them.
Similar complexities arise when determining how "related" that Daeron II Targaryen and Daemon I Blackfyre were, or how related Marcella, Joffrey, and Tommen are. For example, in the last three, they are both full siblings (R=1/2), as well as each other's double first cousins (R=1/4), giving them an interrelatedness of R=3/4. But you can't stop there, because Jamie and Cersei's parents were themselves first cousins, so their children are also each other's double second cousins, summing in another 1/16 to make R=13/16. It’s no wonder they look a lot like each other, as that is considerably more consanguineous than occurs in normal siblings.
Like Aenys and Maegor, Daeron and Daemon were much also more than mere half-brothers. The official story holds that Daeron's parents were Aegon IV and his sister, Naerys, and that Daemon's parents were Aegon IV and his cousin Daena (and thus necesssarily also Naerys's cousin). The Blackfyre claim is that Daeron's father was really Prince Aemon the Dragonknight, brother to both Naerys and Aegon. In either case, they're very related to each other, but those calculations I'll leave for their own pages.