Yes, you can leave your suit jacket unbuttoned when standing and for sure unbuttoned when sitting, depending on the style and fit of the jacket. Leaving it unbuttoned allows for more freedom of movement and a relaxed appearance.
As a general rule of formal wear etiquette, a jacket should be buttoned when standing and unbuttoned when seated. Of course, there are additional rules too. On a two-button jacket, only use the top one when it's time to button up.
