Cassandra Sandsmark is sidekick to Wonder Woman and daughter of Zeus, a demigoddess blessed with great strength and speed, and also weilds a magic lasso. She is sometimes-leader and a prominent member of the late Young Justice and the current Teen Titans.
Cassandra first interacted with Stephanie in Young Justice, when Stephanie, along with a plethora of other teen heroes, came to help the team avenge the deaths of one of their members (Empress) father by attacking Zandia. Cassie was leading the team at the time. The two of them presumably were introduced to each other by Tim, though this is not certain.
In Young Justice #51 when the team was trying to escape from the pulsing earthen mound that was Lady Zand, Wonder Girl yelled, "Get clear! Get clear!" and Spoiler replied while leaping agilely out of the way, "We're working on it, we're working on it!"
During Stephanie's absence, Cassie had a brief relationship with Tim, though she broke up with him before Stephanie returned.
Tim mentioned Stephanie to Cassie as one of the people he'd lost in Teen Titans #50.
In Teen Titans #62, Tim mentioned to Cassie his girlfriend had come back from the dead. Cassie gasped, "What? You mean Spoiler? She's--" and Tim said, "Yeah, Stephanie's alive." A befuddled Cassie said, "Aren't you relieved?" and Tim responded by stammering a lot about all the things he was going through until Cassie forced him to calm down and sit on the bed.
The two girls met again when Stephanie dropped by Titans Tower. Cassie had mentioned in a team meeting earlier that part time members were a good idea, because Spoiler, for instance, would be in Gotham a lot. Tim told her Stephanie wasn't there for that and Cassie replied, "She isn't?" but the subject was changed before Cassie could get answers.
She talked to Stephanie face to face (well, mask) in the training room, kindly but awkwardly saying, "So, you're alive. That's great. That's really great." Stephanie replied, "Yeah, I kinda like it."
Cassie met Stephanie again as Spoiler was coming out of Tim's room, pulling her mask on. Steph reacted awkwardly, saying hi and after some fumbling saying she was just going to one of the guest rooms. Cassie said, "Okay. 'night then," with a friendly smile and wave.
Cassie then entered Tim's room, and after some conversation where Tim was increasingly awkward, Cassie asked Tim what was going on and what it had to do with Spoiler. Tim said that Stephanie is with him for moral support, and that because of Batman being missing and the troubles the Titans are going through, he wanted to quit and hand the leadership over to Cassie. She told Tim she could handle it, to do what he has to do, and hugged him goodbye.
Tim departed with Spoiler in the Bat-Plane, leaving a melacholy Cassie at the Tower. However, the girls were kind to each other in their interaction, opening the door to them being possible allies in the future.
Issues with Cassie talking about/interacting with Steph: