I'm similar to Fiona - 4 years between me and my sis. Once we were both mobile we were still close enough in age to play together and have the same group of friends. When she was really little I was big enough to play 'mummy's helper' and not be jealous of the attention given to the baby cos I was a "big girl"
The only downside was whenever we fought my mum would intervene and tell ME off cos I was "older and should know better!" regardless of what age she was!