Yeah, I hate that portrayal of her. I think she would threaten (one of the Kanzaka interviews says a likely homecoming would be Lina walking into the kitchen to see Luna sharpening a kitchen knife), but I don't think Luna has or would ever physically abuse Lina just out of spite. I think they are normal siblings and Lina says even back during the Rezo arc that certain reactions she has is because of her sister. But, this is also the same sister who told her to go out and see the world - that's how Aqualord begins actually with Lina recounting that Luna told her to go see the world.

So, I think a lot of it from Luna is generally an act and Lina's own greatly magnified fears that she will let her sister down.