You could buy your first bub a baby doll with accessories like nappies, dolly bath etc so they can tend to their baby doll in parallel to you caring for the baby. Let them be as involved as possible - eg helping pass mummy things, participate in bath time, cuddle baby with mummy's help, etc

I would encourage your child to be around and get used to other babies so they feel familiar with what a baby is like and what a baby can and can't do before your bub arrives.

Make sure you have a baby carrier/sling so that you can have hands free when you take baby and toddler out together for activities.

Encourage play-dates to keep the older one occupied so the focus isn't always on baby.