S.P.
Here is a fun idea for all the children.
Get some plain onesies and T-shirts.
Let each child use fabric paints to decorate a onesie for the baby and a T-shirt for him/herself with a "sibling" theme.
Like "Big Sister/Brother"
"Little Brother/Sister"
"I (heart) You"
"(name)'s Little Brotjer/Sister"
handprints, potato prints, or other print designs etc.