I am always struck by the indifferent way that John treats his stepmother and half sisters.

We don't have an exact age for him. We know he married Fanny when he was 'very young'--soon after coming of age. They have a four-year-old son, so we might assume they have been married for five years or so, making John 26 or 27. This would mean that he was only six or seven years old when his father remarried. There is no mention of him having been sent to his mother's people (like Frank Churchill for example, so presumably he grew up with the second Mrs. Dashwood as a mother figure, and in the home with his sisters. Unless he was away at school a lot when the girls were young, he would have played and interacted with them. Seven or eight years older is not an insurmountable gap. My youngest is close to his older siblings who are that much older than he is.

I do find him very cold-hearted to be so almost indifferent to people he grew up with. We don't know how old he was when his mother died, but his half-sisters and their mother were certainly part of his life for much longer than she ever was.