Say that we had one father, say one womb
(Curse to my joys!) gave both us life and birth;
Are we not therefore each to other bound
So much the more by nature? By the links
Of blood, of reason? Nay, if you will have't,
Even of religion, to be ever one,
One soul, one flesh, one love, one heart, one all?
    ('Tis Pity She's a Whore I.i.28-34)

Translation: "Hey, if you think about it, it's only logical that I shack up with my sister."