I know you said you had family meetings. But.. its time for another one.
Time for the ENTIRE family, to sit down and have a Family meeting.
Make it a monthly thing. Liked or not.
Use the time to iron out problems, talk it out, come up with Solutions, to emphasize that they are the kids and YOU are the parents, to allow them to give their take on things too, etc.
Also discuss what is coming up for the family, budget, responsibilities, and anything THEY need to contribute to and chores etc.
It has to be done, together.
Emphasize, what "family" is, and your expectations.
have them write down everyday, THREE things that they did for each other or you/Hubby, FOR the family.
I would not reward them with things or money.
Because, these are intrinsic parts of character and family, that a child needs to learn.
Kids will fight.
But if it was like me and one of my siblings, it will not get better, unless, the ENTIRE family is on the same page.
IF, one sibling is CHRONIC in causing trouble, then maybe you even have to get Counseling for that child.
One of my siblings, constantly picked on me and then lied and/or manipulated my parents about it. Real conniving sibling. It was chronic. It is then therefore, TOXIC to the other sibling.
So.. you NEED to recognize.. .if this is normal sibling problems, or one of them really having, a behavioral issue.... that needs addressing.
So, for me, that is why I am suggesting this. Because, with that sibling of mine, it was not normal sibling fighting... she has a real problem against me and had other issues.
Instead of having family meetings about 'respecting' each other and since they do not listen to 'lectures', make the meetings about what YOU want them to do and not do. Make "laws for the family"... and then consequences. Even putting it down on a poster-board, tacked up in the kitchen.
Since they know, that these family meetings are futile for you and Daddy... then sure, they KNOW you will give up on it.
So don't give up on it.
In sibling fights... there is usually one that is a "Bully"... and the other one is being constantly hammered and because the "Bully" sibling is naturally more manipulative.. then that one will manage to manipulate the situation and their 'blame' to the other sibling.
So you have to be, smarter than them.
What you may have is a bully and a picked-on sibling.
OR, you have a sibling that is taking of the pretended position, of always being "innocent" when they darn well know, they are not.
AND, your kids KNOW.. that you and your Husband, are on opposite sides. Not on the same page about it.
THAT is the problem, too.
You and Hubby, have to just tell them the way it is.
And you can't let them manipulate you or pit you against your husband and visa versa.
Perhaps, you should ALL, go to Family counseling with a Therapist.