Highest sibling but though not visually not on top

Hi guys,
i dont know if i understood the sibling system wrong but i thought that the highest silbing is always on top of the other childs also visually. I have a custom Drag and Drop system with Dropzones as parents. Now i want to set the last draged always on top of the other obejcts visually. And when i drag them they should be on top of basically everything in the game. I do that with transform.SetAsFirstSibling and they also are the first one in the hierachy then but not visually. How can i do that?
Thanks a lot

Last sibling has the highest Z-index by default, unless you override it.

In order for your sibling to render on top, you should make it last sibling: