In Greek mythology, Aphrodite (goddess of love and beauty) and Ares (god of war) are indeed siblings, as they are the children of Zeus, the king of the gods, and Hera, the queen of the gods. However, Venus is not a part of Greek mythology; she is the Roman equivalent of Aphrodite. The Roman gods were often directly identified with their Greek counterparts, and their stories and attributes were very similar.
In Marvel Comics, the relationships between these characters are somewhat different, as they draw inspiration from various mythological sources and reinterpret them to fit the comic book universe. In Marvel Comics, Aphrodite and Ares are still portrayed as siblings, similar to their Greek mythology counterparts. However, Venus is sometimes depicted as a separate character, and her background may vary depending on the comic storyline or the era in which the stories were written.
In some Marvel storylines, Venus is an actual goddess, similar to Aphrodite, while in others, she is portrayed as a superhero named Venus (also known as "Aphrodite IX" in some comics), who is an artificial being created by advanced technology rather than a deity. These interpretations and connections can change over time as different writers and story arcs explore the Marvel mythos.