I am an adult in my mid twenties and I have an relationship issues with my teenage sibling and parents?

I took enough child development and psychology courses in college to know that birth order has a lot to do with our issues but I believe this goes beyond that. My 15 year old sister has a horrible attitude problem. She's very disrespectful to both of my parents and they don't seem to stand up for themselves. Most of the time they excuse her actions because she's "just a teenager" or because she gets good grades. Because of our age difference I was very much a third parent to her while she was growing up. I was much more involved in her life than my parents were, even going as far as being the one who attended her school conferences. I naively believed that she would be more appreciative of me as she came into adolescence. My husband, 18 month old toddler and I recently had to move back in with my parents due to some financial difficulties. I am very grateful to them and realize I'm lucky they are in the position and willing to help us get back on our feet. I have been clearly told by both of my parents that I cannot discipline my sister or judge how they chose to discipline her. That's fair she's not my daughter but I do feel I should at the very least respected. Today in front of a group of family and friends I asked for my sister to check on my little girl for me and her response was "you're her mom" I took the bait and argued back when I knew I shouldn't have and told her I was NOT her mom when she was a child but I was still very attentive to her as a child. I know its pointless to argue with a teenager but this has been building for sometime now. I've talked to both parents on seperate occasions and they have come to the concensus that I am too hard on my sister and am too sensitive in general. My husband is my voice of reason, he tells me to ignore my family when they behave in unreasonable ways but I have done this so many times I feel like I'm at a breaking point and don't know what to do. Any advice? Or similar situations I can relate to? I can't be the only one, right?