During the early 2330s, Sarah became possessed by a Prophet – a non-linear alien species which lived in the then-undiscovered Bajoran wormhole – in order to ensure she met and had a child with Joseph Sisko. As part of this plan, she owned a necklace with the phrase, in ancient Bajoran script, "Orb of the Emissary".
During this time, the Prophet inside her made sure that the necklace stayed in their house.
Eventually, the Prophet left Sarah, and, when she regained control of her body, Sarah left her husband and son without an explanation in 2333 after Ben's first birthday, as she did not really love Joseph but could not explain why she seemed to.
The despondent Joseph Sisko spent three years searching for her. He finally succeeded, but only a month after her death.
The emotional toll of Sarah leaving him and her death leaving no chance for the closure of their relationship devastated Joseph. He later remarried and never mentioned Sarah to his son as he grew up, who assumed his stepmother was his real mother.
Sarah Sisko was played by actress Deborah Lacey.
Lacey commented on gaining the role: "I was familiar with Star Trek. I had been watching since The Original Series. I watched some TNG as well, and I watched DS9, too. I was actually very excited to see Avery Brooks starring in it, and I really wanted to work with him. I had been auditioning for other roles on other Star Treks, so when this role of Sarah Sisko came along, the producers were well aware of me. I believe they had me in mind for her because at my audition I remember one of the producers said to me, “I can't believe you're the Sisko's mom.” I'm still not sure why he said that, though, but I was flattered and it made me feel I had the part". 
Lacey performed Sarah as both Human and Prophet; on how she achieved this, she commented: "Well, it helped that I had played a spirit being before on Roc, the live sitcom on Fox starring Charles Dutton. So, I knew how to play feeling distant from other people. It's a feeling of living in outer space, of knowing you're not a part of this existence, feeling you're in a better place even, removed from their world. I was directed to play Sarah in the beginning as "searching for words," not really sure of the words. So, it's a little "choppy." That's what I tried to do. I relied on that direction, and searched for communication. I imagined what it would be like if I struggled to communicate in a foreign language and only had my senses to work with. You'd find a way to express yourself." 
Lacey liked how Sisko's fate reunited him with his mother in a way: "I'm very pleased that Sisko made it through his journey to complete his destiny and found his mother again. By that episode, I was feeling Sarah Sisko had made that connection of mother and son, and the bond was sealed. She's proud of her son, as any mother would be. It was a journey of love. They found each other, and I thought it was great she wanted him with her".
Lacey commented on what other scenes she would have liked to have seen: "I would have liked to see her as the human Sarah, the wife, the mother. I would have loved to work with Brock Peters and Cirroc Lofton. Maybe some kind of flashback before the Prophet took over that would show Sisko's memories of his mom. And it would've been pretty cool to see Sarah interact with her grandson, just what she looked like being a human being. Also, how she loved him (Benjamin) as a child, where the nurturing began. I'm sure she loved him then, and that memory is shared between them. So, they have it to rely on. That's what first comes to mind".