S.S.
I don't have much experience with 2 childs of different ages. But i do with 2 year old twin girls.
Me and my husband have found out that they want to do things that we do, basically mimic us. So we ask them to help a lot. And my god you should see them, they are so proud of helping us. Like they help their dad unload dishes (spoons, plastic ware) from the dishwasher, get the shoes from the shoe rack to wear when we are getting ready to leave, help mom with opening the cap for the diaper cream, put their dirty clothes in the hamper etc etc. Start with small things. So the trick is to get the toddler to help as much as possible. And giving your 100% attention when the baby is sleeping. This way she feels needed and loved.
I just feel most of the girls have an inbuilt system to take care of others, feel needed.
Let us know how it goes.
Good luck.