P.W.
Just because they are almost grown does not mean your children can't still learn from you. You can teach them, and help them develop useful skills. It's normal and natural to disagree, and it happens in real life with roommates, at work, etc. Developing the skills to resolve conflicts now will help them in the future.
There are 6 kids in my family, and I'm the oldest. We range from 9-23 and a lot of us get into arguments. My mom always points out that she just wants us to get along, and that she loves us.
There are certain "fair" ways to disagree and argue. If you set some ground rules, and tell your children how much you care for them you can help them know when they are crossing the line, like when she is provoking him, when he is being too agressive, etc. He is probably used to her being so young that she looks up to him and now she's asserting herself as an "adult" teenager who thinks she knows everything. As they grow and change they have to re-adjust to each other in some ways.
You might call a family meeting and discuss fair ways to disagree and voice one's opinion, and have them come up with rules for themselves (not each other) like: I will not raise my voice when I disagree with my brother, or I will not use profanity in the house, or I will count to 10 before I talk when I'm upset, etc. Encourage your children to get along because they will be there for each other for the rest of their lives. Tell them how you really feel; that you care about them, you don't want your home to feel like a war zone, and that you want your children to act like civilized adults and set a good example for the 5 year old.
What you say will be more meaningful if backed up (or vocalized) by Dad.