11 parts Ongoing Viserys dies a month after he announces his decision to marry Alicent and a mere three weeks before they would have been married. However, before Rhaenyra can take the throne, Otto announces that Alicent is pregnant with the new King and so Rhaenyra, as a mere princess, cannot sit the Iron Throne.
In this story, the Faith follows a very Biblical style view on engagement and marriage but IS NOT 100% following the Biblical concept. It's BASED UPON A SIMILAR PREMISE. Basically, once a couple becomes engaged, they are "married" just without the official wedding and would need to get an annulment or divorce in order to separate. This is the premise Otto is going to use to declare his grandchild as being a legitimate heir. Things are going to seem a bit murky, possibly, at first, but I WILL be fully explaining things further in the story. I just hope you will choose to follow the story long enough for the details to fully be explained, and different precedences to be brought to light in this AU version of Westeros.
Alicent and Rhaenyra are 16, Laena is 15, Laenor is 14. Alicent is only a couple months older than Rhaenyra and both are "of age".