Then take Yuki in to get a complete examination. There is some reason as to why she's attacking both cats. Are there any new cats around in the area? Is she in heat? Has she been using the littler box properly and regularily? Have there been any major changes in her life lately? Have you moved lately? Have you tried Feliway or any other pheremone based product that may help calm her down. You need to rule out all of this before you can begin to have ideas as to why Yuki is attacking.
And please do not scruff a cat to pick it up. Use your hand under the belly, one hand on the chest and pick them up properly. You can do a ton of damage, including internal bleeding, by scruffing a cat.