]He seems to be the only one who actually expects him to do something for his sisters

Interesting that you point this out, specifically, as I hadn't thought about it very much on previous reads.

Why is it that Mrs. Jennings and Sir John don't seem to wonder about John's relationship with Elinor and Marianne? Mrs. Jennings, the lady who notices everything, must have realized that the Dashwood sisters receive next to no attention from their brother and his wife. Why does she not say anything about it? (Is it because John is so wealthy?)

On more than one occasion, in conversation with Elinor, Lucy reveals an assumption that strong family ties exist between the Dashwood sisters and their brother. Lucy assumes that Elinor has influence with John (help Edward get the living at Norland), and that the Dashwoods will be staying in London with John and Fanny.

But the best example is Fanny's friend who assumes that the sisters are staying with John and Fanny on Harley Street. Apparently Fanny would rather keep up appearances by including Marianne and Elinor in the outing, even though she resents having to send her chariot for them, than have it get around that her husband's sisters are not even invited to stay with them.