Considering that the Romans would have borrowed the relationship between Hera and Aphrodite from the Greeks for their own versions of the same goddesses, respectively in Juno and Venus, then Juno would be Venus' stepmother, since Juno was the wife of Jupiter, and Venus was the daughter of Jupiter by the Oceanid Dione. (Dione was a first cousin of both Jupiter and Juno.)
The Romans, however, would likewise have imported the other version of Aphrodite's coming into existence, in which she was a by-product of the mingling of the severed genitalia of Ouranos (the Greek sky-god whom the Romans called Caelus) with the salt-foam of the sea, into which they [the genitalia] had been cast by the Titan Kronos (whom the Romans called Saturnus, the father of Jupiter and Juno). In this case there would be no direct familial relationship between Juno/Hera and Venus/Aphrodite, unless one considered Venus/Aphrodite a sort of daughter of Ouranos/Caelus, in which case she would be an aunt of Juno/Hera.
Additionally, the Romans would also have incorporated the story of Aphrodite having been married to Hera's son Hephaistos, which thus would make Venus/Aphrodite the daughter-in-law of Juno/Hera. I think that in Roman mythology there was also a closer connection in the story of Venus' love-affair with Mars than there was in the Greek original between Aphrodite and Ares, perhaps to the extent that Venus and Mars were married at some point as well. Mars was also a son of Juno, so this would again make Venus a daughter-in-law of Juno.