A few days ago a woman on a Yahoo group I belong to wrote asking for advice about her teenage daughter's attitude toward her younger siblings. After I responded with my thoughts, several people wrote and thanked me for my ideas. Since they seemed to be helpful, I thought I would share them here.
I'm not sure I can help you with your current situation with you oldest daughter, but I will share how my husband and I train our children who are in their teen(daughter 16, son 14, and daughter 11). First, I just have to say that we don't ever use the term 'teenager' when referring to our children. We feel that teenager has such negative connotations and even just using the term gives a tacit 'pass' to obnoxious behavior. Along the lines of, 'Oh, those teenagers! What can you do?' We give our children a choice of being a young adult, which means that they behave in a mature way, are helpful, and consequently have privileges that the younger children do not have; or of being a child. If they are behaving like a child then they also lose the privileges that a young adult would have. We have discussed this with all of them and they understand the difference.
We also focus on tone and attitude...a lot. It is so easy for this age to fall into habits of being sullen, or snide, or condescending to others around them. I call each of them on poor tone every single time I hear it. We also do not allow them to roll eyes, stomp off in a huff, or to pout in one's room. If one of them says something mean to another I point out why what they said was interpreted as being mean (or sassy or whatever tone I hear) and ask them to rephrase what they said. If it was something mean to a sibling, they must say three nice things about that person. I will sometimes ask the (now) child to do something nice for the injured party or for them to take over the injured party's chores for the day. My goal is to make that kind of behavior completely unworthwhile for the older child. It is going to cost them in a major way. The key here is that we do this every single time, and for all relationships. If a child stomps away from me, I have them practice walking away calmly. Sometimes we do this multiple times. All the children know what the consequences will be for this type of behavior. If they don't comply with the consequence immediately then we practice the correct behavior more times and I often have chores (often involving physical labor) that can be given the recalcitrant child as well. I also (try) to never react myself over this behavior, but keep it very matter-of-fact. If you do this, then this happens. Every single time. I believe that young adults can be in control of what they say and they know what they can get away with.
If I have a child who is needing correction more than usual, I need to ask myself why this is happening. Have I spent enough (positive) time with them? Are they worried about something else and poor behavior is they only way they know to deal with it? Sometimes a string of bad behavior is not about the presenting problem, but only a symptom of something else going on. I really think detective should be added to the list of parenting duties. And if I'm completely truthful, sometimes bad behavior on my children's part is mirroring my own poor attitude. Nothing like seeing my children mimicking my own bad behavior/attitudes to pull me up short. Ugh.
I tend to be slow to offer advice about raising adolescents since my oldest is only 16 and I have yet to raise a child to functioning adult. But, I thought I would share about how we deal with the attitude issue. It has worked for us pretty well and I don't have to deal with these issues very often. I hope you can work through this with your daughter so you can begin to enjoy her again.