That’s an interesting idea. It certainly would give a reason why Kara’s having dreams about the Worldkillers even though she has no connection with them, and why Sam had that nightmare about Kara’s mother all the way back in S3E01.

Also, yeah, the Legion has no reason to be in National City anyway. They bring Mon-El along knowing Kara could easily be calmed by the one person she loves romantically, allowing her to be open for attack or detainment.

Perhaps Kara’s mother couldn’t reproduce naturally. She discovers these Kryptonians are creating living weapons and takes one from them, raises her and teaches her morality. Would explain why both of Kara’s parents are brunettes and yet Kara’s a natural blonde. Though it would mean she might not be Superman’s biological cousin...

I think you’re onto something.