That's the downside to being the more mature one in a group. On the one hand, you're mature. You hold yourself and others accountable for things. As you should. On the other hand, it makes working and co-habitating with less than helpful people frustrating. It won't last forever. Even if people aren't willing to change, you will eventually be able to move out and create your own space. I'm nearly 25 and I had to learn (in a very painful lesson) that you can't change people. You can't always change what they do to you or how they treat you. The only thing you have control over is yourself. You can ALWAYS change the way you react to others. Past that, you've done what you can, right? You can be happy that you've at least made the effort and done what you can, even if things aren't exactly as you want them to be.
As for the 11-year-old, just give it time. It's a typical part of lifespan development. Once kids hit puberty, they start questioning everything from authority figures of any kind. They begin to realize that adults aren't always right and they start wishing for more control as they develop their identity. It's normal and every child does it to some degree. It means she is developing normally no matter how frustrating and intolerable it is.
My sis is 15 and she's starting to calm a bit. My brother finally figured out that responding to her arguments and continuing to argue with her just escalated matters and he's learned how to handle her constant questioning and arguments. Much to everyone's relief.
And I understand about the fish thing. My grandparents were very critical about how I cared for my rabbits. They thought I was spoiling them when I was really just giving them the necessary things that they needed. We had very different views of animal care. Same with my parents. I removed myself from my grandparent's house/state as their differing views created stress that aggravated a medical condition. I still have my parents to contend with, but I've learned to just remind myself that I can only do what I can do for now and work my way to eventually creating my own space where I get to be queen.