Only Fools and Horses, Series 4 (1985)

Rodney:
(about Debbie) I've just met the first girl in my life who really means something to me, and it turns out to be my bloody niece!

Del:
Alright, Rodney. Come on, that's why I had to tell you, you see, 'cos this sort of thing ain't allowed - it's... well, it's incense! Say you had got married to her - you can see what sort of confusion that would have led to, I would have been your father-in-law!

Rodney:
Bloody hell!

Del:
Your mother-in-law would have been your aunt, your wife would have been your second cousin - God knows what that would have made Grandad - the fairy godmother I should think.