idk it's like something to bond over? they messed with both of us let's kick their ass? you are useful to me and we both want these people gone so i guess i won't kill you yet giving us time to form a strong, complex bond? but yeah i get what you mean that they both already have potential to really care for each other. i totally dig the idea of them being siblings! don't be so hard on yourself your ideas are rad! ^.^
ok ok so they be siblings. big bro and little sis. jack's first interaction with his long lost little sister was to find her hot and hit on her. nice. well actually, it was to try and kill her lol.
ok so here's my idea for lexa's backstory: jack was taken as an infant, right? so how about for one reason or another they snagged his mother, too. they tortured and ran experiments on people as evil organizations do, permanently screwing up jack's psyche in an attempt to make him a killer or give him powers or whatever. while they were doing that, they were also running tests on momma, as well as maybe other women to see if giving powers to a developing embryo was more effective than inducing a change in children.
(this likely means that lexa and jack are half siblings since the scientists weren't super concerned about who fertilized what eggs as long as there were results. i'm p sure lexa doesn't know the parents she grew up with aren't her birth parents, and her backstory hints that she grew up in a french-vietnamese household. so she's likely some sort of mixed-race to look like she at least plausibly came from her parents. idk tho, east asian/white kids rarely look like either parents. i know from experience, lol. she'd probably kinda look like her little brother though, who was the parents' birth child)
so something finally stuck when lexa came around and she was the first live subject to be born with powers. scientists were super stoked. then they used her genes as the basis to give everyone else their powers. somehow someone got baby lexa out her parents adopted her, and all the shit went down back at the lab with the other kids and jack.
how does that all sound? feel free to suggest alterations.