Ras al Ghul's other daughter finally made her Arrow debut in the show's winter premiere, "Who Are You," but it wasn't in Star City. Talia al Ghul, Nyssa's sister, walked into Oliver's life at just the right moment, not in the present, but in the past. Talia al Ghul made her Arrow debut in this week's flashback Bratva storyline.
Most of this week's Arrow took place in the present, where Laurel appeared to have risen from the dead and the threat of Prometheus was very, very real, and the flashbacks mostly consisted of Oliver being beat up by a member of the Bratva. Only at the end of the episode, when Oliver was about to be shot in the head, did Talia appear, breaking through a wall armed with a bow and arrow, looking for Oliver. "Hello, Mr. Queen," she said, "I've been looking for you." Whether or not Talia knows Oliver from his Starling City playboy days, his days on the island, or his misadventures in Hong Kong, we don't know, but whatever she wants from him, it probably can't be good. (Seriously, when have the al Ghuls brought Oliver Queen anything other than misery and a few stab wounds?)
Given the fact that Talia saved Oliver's life, we can assume that whatever she wants with him, she needs him alive. But, we also know that whatever Oliver and Talia do together in the past, Ras al Ghul is ignorant of it, and it doesn't particularly seem like Nyssa knows that her sister knew Oliver either.
Talia al Ghul could be a major ally for Oliver, or she could be just one more scar on his back.