My youngest was born when my eldest was 21 so a big gap, too. But dd's spoiled rotten and has the added joys of gaining two lovely sisters-in-law. One day she might even have nieces and nephews to irritate play with. All her cousins bar one are much older but as they've started having babies themselves, she rather enjoys being The Big One instead of the wee one. I really wouldn't worry, I think families have a fine way of working themselves out.
My little brother was a surprise 17 years after I was born. He is the youngest of 4. He is spoilt rotten by his older brothers and sisters and we are all very close. He spends a week every summer holiday from school vsiting me and we do some cool day trips together. Now he has 6 neices and nephews too (one of them only 6 years younger than himself) so plenty of playmates at family do's.
Dont worry about your little Ds, he'll have the best of both worlds!