Better, for uncleanness; ἐπὶ denoting aim or intention. The intention is viewed as the basis of the act (ἐπὶ upon). Comp. Gal 5:13; Eph 2:10.
In sanctification (ἐν)
Note the change of preposition. Sanctification is the characteristic life-element of the Christian, in which he is to live. Comp. in peace, 1Co 7:15; in hope, Eph 4:4.