I'm sure medieval incest has been done but I wanted to do a story myself about a brother and sister. I know back in the time there were a lot of incest marraiges to keep royalty going. I thought that would be interesting to do. Any ideas?
In the middle ages, they tried to avoid ... oh, crap, what's the word? There's a word for it, for too-closely-related. Consanguity? Something like that. Means "common blood" or "shared blood" or something. In order to marry a relative, said relative either had to be distantly related to a certain degree (second cousins or so), or a dispensation had to be granted. It would be pretty unlikely for a brother and sister to be married.
Cousins, yes. Half-sibs, possibly. And the royal families did get pretty closely intertwined -- that's where the term "King's Evil" for hemophilia came from; they were drawing from a fairly limited genetic pool and so once a flaw like that entered the family, it could be hard to get rid of.
Which isn't to say that there wasn't plenty of sibling incest going on; just that it wasn't often found in marriage. For that, you need to visit ancient Egypt or the Incas They took the concept of 'royal' blood a step further, to 'divine blood,' and didn't want to dilute it outside of the family.
I agree that historical accuracy would not favour sibling weddings in European royalty- cousins very common, but not sibs.
One twiist might add in the prevalence of 'bastard' children- often sired by ther King with one of the lesser noblewomen serving as ladies in waiting. These bastards often ended up in the church or the army. Perhaps one becomes the lover of the princess, his half sister?