Advice: Response to Child's Question About Sibling's Disability
Use age appropriate and positive language. I would stick to something simple, as you have been. "Everyone is different," but stress also that being disabled is not wrong or bad. It just means her baby brother will do some things in a different way. Then talk about all the ways she and her brother are the same. Show her positive examples of disability from TV shows. Daniel Tiger's Neighborhood has a great episode, appropriate for her age:
There's also this lovely song by a little girl on Sesame Street, where she sings about her wheelchair:
Basically, it's super important that she understand that there are lots of kids and babies out there who have all kinds of disabilities, and that is totally okay. That kids and babies have lots of things to help them like wheelchairs and braces and crutches, and glasses so they can play and be happy just like she can. Hope this helps!