Or Luke could end up having numerous siblings. Luke's family tree could go something like this:
Padme=Obi-Wan + Anakin
| |
Luke Han=Leia Chewie Jabba
Luke could ask Obi-Wan if he is his sister, to which Obi-Wan could reply, "No, I am your father."
You could place this scene in Episode V, before Luke leaves for Bespin, so that when Darth Vader tells Luke that he is his father, Luke could reply "That's not true! Obi-Wan told me that he was my father!..."