T.S.
So how is it going now?
I think my profile says it, but if not I have 20 ds, 19 dd, almost 15 ds.
My kids are not allowed to talk bad to each other--never have been. This stuff starts in preschool. To curb it right now I would cut off everything and start from scratch.
In other words if they can go a week with no fights/sarcasm/mean teasing/snickers, etc. then they get to use the computer for a day or two. I mean it is tough but that is what it takes to motivate teens. They seem to think it is their God given right to have everything their way. It is our job to show them it is not. ;-)
I kept a kid for 1.5 years (13 yr girl) and she was raised fighting with sibs. I told her in my house that does not happen. You can't be nice, then you don't need to talk, be on the computer, be on the phone, use the tv, get a ride to friends, go shopping, go out to eat, anything.
She still was not the nicest, but she learned 'shut-up' is not a word around here. Interrupting is totally ignored and even penalized, etc. Your house, your rules. Even if the older one does not live there she needs to follow those rules while she is visiting under your roof.
Oh, and she is not the one that needs to demand respect. She needs to see if she is giving respect! :-0 Age has nothing to do that with that! All people need to evaluate that if they want to be respected.
Good luck,
ts