Why is everybody so damn mean to Nami? Sure, she is difficult and obviously wallowing in self-pity, but that isn't a good reason to treat her like lowlife. None of her sisters even once wonder why she is so depressed and simply kick her around as if she is some sort of nuisance, instead of family, which really only makes things worse. It looks like a very contrived scenario to make Nami turn on everybody later on.
And yes, Nami is right about Akiha's preferential treatment, no matter how you look at it. What would be the trouble for the sisters to spend some time to just talk to Nami about what is going on? She may be bitchy, but she is still family!
What's worse is that we're just meant to accept, for the longest time, that she's past redemption.