My sister was 13 when I was born. She left to go to uni in another town when I was 5. I was the annoying little sister, so she could not really be bothered with me much. (Not your usual maternal girl, my sis) I got used to only seeing her for Christmas and summer holidays. Well, saying that she usually worked somewhere in summer, so I saw her very rarely. She moved back to the parental home when I was 16, looking for work in our home town. It was a disaster. We were fighting and arguing daily, although I was a pretty good and calm teenager, she brought out the worst in me. I moved out 2 years later, and we slowly started developing a relationship when I was in my twenties.
You can never predict how the relationship between your children will be, regardless of age gaps, so you shouldnt worry.
I should also add that it seems to be the norm on my street to have large age gaps.
Family one: dd1 23, dd2 8, ds 4 (now father has sown his oats in younger fields and moved out, though)
Family two: dd1 23, ds1 19 and ds2 7
Family three: dd 24, ds 8 (but these children do not have the same father)
All these have been happy babysitters to their younger siblings, and still are. Only the daughter in family 3 has a live in boyfriend, who is the hero of younger brother, and this young man is very much present not just in this boys life, but in the life of all the boys in the street.