Sisters not so much imo (Kara called her a sister but I think that was put in to further whatever plot they have planned with the Kara/Lena/Sam friendship. Plus the writers will put in anything that pushes their plot along, not caring if it’s rushed or forced), but to me they are definitely just platonic.
I know shippers like to see Kara as being utterly in love with L*na, she would die for her (pretty sure Kara would die for anyone), would do anything for her, needs her a lot, and I’ve seen some name her as the second most important person to Kara. But in canon that’s not very true (except for the she loves her, because she does but she’s not IN love with her. She loves her like she loves Winn and James, but nowhere near as much as she loves Alex). I mean, this might just be me but, I have trouble even seeing them as best friends sometimes. Like in s2 most of the times Kara went to talk to Lena was when she needed something, then suddenly after the hiatus they are bff’s and they love each other but I feel like we never truly saw their friendship develop (same has happened with Kara and Sam). Also, most of the friendship feels unbalanced because all we see is Kara supporting Lena, Kara is there helping her development but we don’t see much of Lena supporting Kara (we’ve had one or two that I remember) or helping her development. It’s why I have a hard time feeling invested in the friendship because it feels to me too much like Lena benefits more from it while Kara as a character doesn’t get much in return other than another friend (and not even one that is heavily involved in the important aspects of her life), and I am here for Kara primarily (and Alex ofc). Perhaps this’ll change once she finds out Kara is Supergirl.