Actress Kate Hewlett has been cast in the role of Jeannie, Rodney McKay’s sister, in an upcoming episode of Stargate Atlantis, writer-producer Martin Gero confirmed to GateWorld today.Hewlett is the real-life sister of “McKay” actor David Hewlett.Kate Hewlett will guest star in “McKay and Mrs. Miller,” in which Rodney is reunited with his estranged sister on Earth. Beware of spoilers for the upcoming episode below.”It’s an episode that I’ve been wanting to do for about a year now and just haven’t found the right door,” Gero told GateWorld. “It kind of came out of the fact that I was back in Toronto last summer. … And I saw a play starring David Hewlett’s sister, Kate Hewlett, and she was spectacular. She’s a phenomenal actress, and had a lot of — dare I say — ‘Hewlettisms’ about her. And they have a great chemistry in real life. … But after I saw the play this summer I said, ‘David, I’ve got to do an episode about your sister.'”Though tight-lipped about the plot of “McKay and Mrs. Miller,” Gero did reveal the reason behind the tense relationship between Rodney and Jeannie.”She’s equally brilliant,” he said. “Let me put it that way. But she has kind of left the physics game. She got pregnant, basically, when she was in university, and decided that being a mother was really important to her. She kind of put that part of her life on hold. McKay never forgave her for that. He felt that she was potentially more brilliant than even he was, and feels like the contribution she could’ve made to humanity far outweighs going home and raising a kid.”It’s those two schools of thought meeting. It’s about compromise. It’s about accepting who your brother or sister is and trying to find some middle ground with them.”The casting of Kate Hewlett as his sister is serendipitous, according to David Hewlett.”When we did ‘Hot Zone,’ where [McKay] confesses that he has a sister, it had originally been written as a brother,” David Hewlett revealed to GateWorld. “And I said, ‘Look, just on the off chance, I’ve got a lot of sisters and one of them happens to be an actress. I’m not saying you have to cast her, but just on the off chance, can I say ‘sister?” And they said, ‘Oh, that’s a good idea. That’s fine.'”Hewlett recently co-starred with Kate in “A Dog’s Breakfast,” a film he wrote and directed (story).”I’ve just worked with Kate, because we just did this film together in January,” he said. “And she’s fantastic. And that’s me saying it — I’m usually tougher than anybody on that kind of stuff. And one of the producers actually saw a couple of things she did recently, as well, and said she’s good.”Gero is looking forward to seeing the episode filmed. “It’s a neat story about siblings,” he said. “I have a brother and two sisters. It’s less trouble than McKay’s relationship with his sister. But I think there’s something universal. It’s kind of [about] family and pride. It’s quite a funny episode.”Season Three of Stargate Atlantis premieres in July on The SCI FI Channel in the U.S.