The sibling seat can be placed either in the upper or lower position, depending on whether you use it together with a bassinet or a seat.
With a bassinet, the sibling seat should be placed above, facing the parent. The bassinet should then be placed below, using the lower sibling adapters.
With a seat, the sibling seat should be placed below, while the main seat sits in the upper position.
If you want to have the bassinet in the upper position, use the upper newborn adapter (available separately) and keep your baby closer to you.
Check the product's user guide for a complete list of recommended set-ups.