As scent and hormones are connected and cats cue to scent, it is very likely they know of a pregnancy quite easily. It is cat "manners" (at least among my troops over the years) to let a pregnant queen have first pick of everything and if there are 2 pregnant, the higher "ranked" get best dibs. If your cat is female she may be honoring the rights of your sister by not even trying to compete for anything.

When I was hand raising a kitten, my females did not enter the room where the baby was until I brought her out. The males all strtted their stuff as if they had created her...and spoiled little Belldaonna rotten! When I jave held human babies similar behavior occurred...tho the females all cuddled with the baby if it cried.