If Big Sister or Big Brother is attending the celebration, it is thoughtful to involve them. Becoming a big sister/brother is a huge change at any age and so much talk and attention centers around the unborn baby, it can be helpful to make Big Sister/Brother feel special!
You can create a Big Sister/Brother Kit out of a box or a basket, which would include a few items to help out with the baby: pacifiers if they're the binky-fetching type, books to read to the baby, art supplies to make a card to welcome the baby... you get the idea. You can also include little things just for them - stickers, a big sister shirt or anything really that they will enjoy playing with. They don't care if it fits the theme; they just like getting a gift, particularly when their mom just scored 20 gifts for the baby.
You can also create a Big Sister name tag. In this case, we created a VIP "backstage pass" for Big Sister, Taylor, at her mom's rock star baby shower.

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Email your idea, story and pictures to hotmama [at] lovecheekybean.com!