Wear her when you need your hands free. If they go at her too hard with their hands, catch their hands and guide them to use, “gentle hands!” That’s what we did with both of my boys. If they do end up being a little too heavy handed, tell them, “Gentle hands! When you hit or push hard, it hurts her! Ouchie! We love baby sister and we don’t want to hurt her or make her cry!” Likewise, when they do use gentle hands, make extravagant praise! “Ooh! Good gentle hands! You were so kind to her! You’re such a great big brother! Look! Your baby sister loves you so much!”
This is also how you train kids with dogs or other pets.
I think showing gentleness is good. If they accidentally hit her she will be okay. But maybe get them a baby doll to practice being safe. Keep her off the ground so they don’t run her over. But I think holding her while on the couch and your other kids around touching and playing with her will help