For as poorly as Oliver Queen may have treated Laurel throughout the events of Arrow, her sister Sara Lance didn't fare much better. Despite their supposedly being very close, Sara backstabs and holds secrets from her sister as much as Oliver does, making her no better. This even sees her give up on trying to save Laurel from death, despite all of the other times that she and the Legends of Tomorrow have played around with the timestream.
The result was a relationship that the Arrowverse said was strong, but was almost never shown as such. Sara may have become a good leader and hero in her own right, but she was a terrible sister to Laurel and frankly gets off the hook for it way too easily. Here's how the first Black Canary constantly made her sister cry.
Sara Lance Had an Affair With Laurel's Boyfriend
Back in the day, Oliver Queen and Laurel Lance were madly in love. In fact, it was this love for her that helped drive Oliver to try to get off of the island of Lian Yu. Despite these feelings, both Oliver and Sara would betray Laurel's heart by engaging in a secret affair with each other. This saw Sara sneak onto the Queen's Gambit ship to be with Oliver, and the secret act of cheating seemingly resulted in her death. Of course, this wasn't the case, but Sara would have never made her family think she was dead if she simply respected the relationship that Laurel and Oliver had.
Sara Never Told Her Family That She Was Alive
Sara, in fact, was not dead but saved and trained in incredible martial arts mastery. She doesn't return to Star(ling) City until a while after Oliver has returned and begins acting as a vigilante, furthering the belief among even Oliver that she is dead. Laurel likely blamed Oliver even more than he deserved for her supposed death, something that Sara could have curtailed if she had taken an earlier break from gallivanting about with the League of Assassins.
Sara Constantly Disrespected Laurel Upon Returning
Even after revealing her survival, Sara still failed her sister multiple times. She essentially disrespected Laurel by bringing Oliver as her "date" to a dinner party, only fueling resentment about how Oliver cheated on her in the first place all those years ago. She also lets Oliver in on the specifics of her survival a lot earlier than Laurel or her dad, let alone her many times forgotten mother.
Perhaps the worst way in which she disrespected Laurel was in not trying to save her from death at the hands of Damien Darhk. Sara and the Legends constantly messed around with time and space but simply accepted Laurel's death as being meant to be without question. This was after Laurel went out of her way to save Sara after she had truly been killed herself. This meant using a Lazarus Pit and even having Oliver contact Constantine to save Sara's soul in order to bring her back. Yet, for as far as Laurel went for her sister, Sara never went even half as far for her. The biggest insult may be that Laurel only lives on through Black Siren, her evil counterpart that's not currently starring in any series, whereas Sara still leads the charge on Legends of Tomorrow. Sara definitely loved Laurel, but it's obvious that affection didn't go far enough.