I vaguely recollect learning about this in a Family Law course as part of my law degree (this was early on, before I stopped attending lectures).
The legal term in the UK is
consanguinity .
As I remember, 1st cousins are allowed to marry here, but uncles/nieces (aunts/nephews) are not. But the uncles/nieces (aunts/nephews) ARE allowed to sleep together (and not in the Lexington Brian/Olivia way). That sounds WAY too close genetically to be a good idea, so I'm now thinking I could be wrong, will go look it up...