As a mom to four adorable girls, I very well understand the desire for a great photo of all your children together! I'm happy to include older siblings in a newborn session at no additional cost but I do want to offer some guidelines for a successful newborn session with big siblings.
Bring an additional adult. Dads often accompany moms to the newborn session; mom, a friend, a sister is also great. Occasionally, schedules are such that a new mom is bringing the newborn by herself to the session. If the new mother is recovering well and is able to drive, it's not a problem for newborns being photographed alone. But attempting to bring other children along with the newborn is quite a lot for a mother who delivered just one week ago. When bringing older children, please be sure to have a 2nd adult in addition to mom.
Consider having the 2nd adult take the older children once their part of the session is complete. Dad or Grandma taking the children off to breakfast or home or the library or the park helps keep them from getting bored while we focus on the newborn and gives us a calm and quiet environment for the remainder of the newborn session.
If the older children do need to remain for the session, consider bringing a snack and toy to occupy them for about 2 hrs.
Keep the focus of the newborn session the newborn. A full, outdoor family session showcasing all the relationships within the family would be wonderful at a later date. We don't want to use up the newborn's sleep time on the siblings and family poses. For this reason, I limit the family and siblings poses to just a few.
A note about matching outfits with siblings: Many times, a matching set for the siblings is planned but I have two cautions: Most poses make writing illegible. The adorable shirts saying "Big Brother" may be covered by the baby, or arms. Also, the newborn wearing only a onesie will not be as settled as a newborn that has been wrapped. Photos with the newborn in a onesie will often have baby's arms out and about as baby decides to move. A wrap keeps baby nice and posed, with their hands out of their face.
When working with toddlers and preschoolers, we need to be flexible and patient. They've had some bring changes in their life and sometimes, they simply aren't in the mood to cooperate. I usually can coax at least one image but can't guarantee.
Enjoy some examples of when older sibling DO enjoy posing with their new sibling!