There are two ways to market the Sibling book to every customer ordering a Baby book. Both options are great, as your goal in building your business is to increase the average order sizes and bring customers back to order again.
1. As a combo on the initial order: Give them a discount on the second book if they buy "now".
2. As a sales follow up: Send a follow up email within the first 10 days of their order. You can also create a coupon, maybe call it "sibling"
An example of the follow up email:
Thanks again for your recent order!
I wanted to follow up and let you know that if little Sara has any siblings we have the perfect keepsake gift for them too. The arrival of a new baby brother or sister is an exciting time for the whole family. To prepare your children for the arrival, we created this personalized story to help them understand why new babies require extra attention, and that being an older brother or sister has its perks! Your older sibling(s) will feel special about receiving this book, and will learn from it too.
You can read the sample online
(You would replace the link above with your own link to this product on your website)