i make tumblr themes and it work fine in my brother computer where i make it. But, when i look it in my sister's computer (the monitor is bigger than the monitor of my brother) .. the theme was damage.

Nothing to do with the monitor! Simply a different screen resolution resolution. Set the larger monitor to the same screen resolution as the smaller one and they will both look the same (albeit larger overall)

Make the theme a fixed size (simplest) or redesign it to be a fluid layout.