Photographing a newborn baby with a sibling can be fun but it can be challenging, requires lots of patience and time especially if the older sibling is just a toddler.
I always start the siblings session with some sort of laying down pose on the fluffy rug where older sibling hugging, kissing the newborn brother or sister and next one where sibling is sitting next to the newborn baby (newborn is in the little bed, pram or basket prop). Teenage siblings can pose holding the newborn baby if they feel confident( the newborn baby is usually wrapped then).
For siblings photography I always recommend bring couple of outfits, something plain and neutral, please avoid clothes which are baggy, have stripes, big logos, writing ( example big brother or big sister).
For boys polo shirt, short or long sleeve or just a plain T-shirt with neutral colour pants or jeans.
For girls I recommend a white, cream simple dress or tunic, avoid fluffy, bright dresses, as they take too much attention from the newborn baby.
I always recommend bringing some snacks and some BRIBES for the siblings photo session but only use it when we absolutely need them.