I completely, totally and strongly agree with poisonivy! I would have a full family sit-down meeting and go over the house rules (that you and your husband have already agreed on privately). Make it clear that they apply to everyone in the house so that no one feels singled out or picked on. PoisonIvy's suggestions are great. If you hear her using a rude tone or being nasty to the other kids, take away a privilege. Don't back down, don't waffle and don't let her manipulate her way out of a consequence.

My oldest SS is only 12 so I know I will be heading into this territory too. One thing is that a lot of people (usually people who have rude and horrible teens) just accept inappropriate behavior as "teen" behavior as if that somehow excuses it. Obviously, the teen years are difficult and if teens didn't rebel a little they would never leave home! It is best if you start out with good discipline as early as possible, but no matter what the age is, you can insist that everyone in the household treat each other with respect and kindness.

That addresses the symptoms, but it sounds like this girl might need therapy to work through her issues. It's normal for kids to fight, especially siblings, but it's not normal for a 15 year old to be verbally abusive or nasty with younger kids. I don't know the situation but it sounds like she is having a hard time with the whole blended family thing and her BM sounds like she doesn't help either.

You won't necessarily be able to make her change and become a better person and sister, but you can and should demand respect for your kids and yourself. My SS12 is like a different kid compared to when I first became his SM, but even now, if he has been with BM, he comes over with a lot of attitude and a negative, whiny outlook. I remember being shocked when my H (my fiance at the time) asked him to do some chore and he whined and complained and then got another lecture. Next day, instead of whining his comments were technically polite, but his tone was so rude that it got him the loss of one day's video game time. In general now, he is pretty good, it's my SS6 that is the whiny, manipulative one at this point. SS12 rarely says anything really nasty anymore, but he still does use his bratty tone with us if he's been with BM for a while and we have to nip it in the bud immediately. If I pick him up from school and he's in a bad mood, I tell him that he is allowed to say how he feels and why he's upset, but he knows that he is not allowed to call people names, swear, mock or tease. He is NOT allowed to whine on and on and blame everyone but himself for his own mistakes. If I ask him how school was and he answers in a nasty tone, he gets a warning and if it happens again he loses a privilege. On days that we get the boys after they have been with BM for her weekend, they are usually completely strung out on junk food and sugar, in the habit of fighting and hitting each other and very rude and nasty to adults, including strangers and friends. If I pick them up and it seems like it's going to be one of those days I pull over and park and then remind them that even though it's hard to switch, they are with us now and have to follow our rules. Most of the time I just have to give that little reminder and they snap back into our mode. On bad days, they get a warning in the car and on awful days they both wind up going straight to time out at home and not getting to watch tv or play video games. They obey us really well and it's because we are always consistent. Their "punishment" isn't to make me or H feel better or to vent our anger, it's for the boys and so they will grow up knowing basic manners and respecting people. My SS12 is generally extremely obedient, but tone is still a big problem with him. He walks all over BM and is incredibly rude to her, but she puts up with it and is rude to him as well. That may fly at BMs, but we're the kind of family that raises well-behaved kids, not any of that "boys will be boys" crap. Not in our house! I don't care if you're 15 or 115, I still won't put up with anything other than basic manners as a minimum.

Figure out the stuff she loves to do and have losing them (temporarily- we usually go from a day to a week depending on the infraction) be a consequence for her behavior. If, after explaining the new rules to her and the entire family, give her one warning if she is nasty or rude to anyone and then take away a privilege (phone, computer use, TV time, friend time etc...)

If you stick to your rules and are in agreement with your spouse, this will work for you even if it takes a little time (kids rebel at new things, especially new rules)

Oh, and one last thing! You only have about three years left 'til she's 18!!