DD1 was born when DH's half brother & sister were 6 & 9. They were 4 & 7 when we got together & DH moved out. There are 14 years between DH & his brother, & 11 between DH & his sister, but they get along great. Beck - his sister comes over here all the time to visit & stay the night. She's 14 & is also my usual baby sitter. Dave - his brother is 11 & in year 5 at school, so I know that he is there to keep an eye on Jaz for me. He treats her like a little sister. Torments the crap out of her, but won't let anyone else do it.
I really like that when Jaz & Dave are older he will be perfect to chaperone. He can look after her, but he's not so old that it will be embarassing. They might have very different views on this then, but too bad.