As far as Aphrodite was concerned there was no comparison between her and her half sister Helen. However Paris proclaimed Helen the fairest in the land. A short time later a wedding was announced.
Whomever gives me the best bribe is the fairest!
Eris was not invited to the wedding. So in result Eris makes a surprising and unwelcome visit to the wedding. Looking to cause chaos Eris throws a golden apple and says this apple is for whomever is the fairest.
....And that is my bribe.
Hera, Aphrodite, and Athena all grabbed for the apple. Its mine insisted the three goddesses Aphrodite loudest of all. Zeus decided Paris would choose who the fairest was.
Paris was called upon by Zeus and he came. He being a foolish man said to the goddesses whomever gave him the best bribe would win.
The three goddesses made their bribes. Aphrodite being wise with foolish men won due to her clever bribe.
Aphrodite's bribe was Paris would be enchanted to her half sister Helen and his brother's would be jealous of him. Aphrodite was now the fairest in the land. The end.