i only have one kid, but my siblings fit the pattern. my one sister and i are only 18 mos apart, but i am a full 7 1/2 years older than the youngest in our family. we weren't really meant to be that far apart... my 3rd sister died as a young child, so we were supposed to be 1 1/2 yr, 2 yrs, and 4 yrs apart. anyway, like some other people have said, we loved having a new baby around, and i know it was much easier on my mom to have two older girls who were willing and able to help out, and at the very least be a bit more indepedent.
my closer sister and i were too close in age and too different in personality to really get along as teenagers/adults (VERY competitive), but we were really close as kids. my youngest sister and i were too far apart in age to really interact much, and there wasn't any intensity to our feelings about each other. we love each other and enjoy being sisters, but it's hard to feel very strongly about someone who is in a totally different stage of life (does that make sense?).... when i was in highschool, she was just a little kid that i could benignly pat on the head and ignore if i wanted too. now that i'm married and have my own kid, i have to make a real effort to stay involved in her life in any meaningful way.
i think it is nice for her, to have two older sisters that have kids and families, but still get all the benefits of being an only child (which fits her personality)... she gets to travel, her pick of bedrooms, choice of activities, much more freedom etc. all of which come from having older, more experienced, relaxed and financially stable parents than they were when we were young. if she weren't the type to really thrive on independance and solitude, i'd feel sorry for her that she missed out on all the chaos and fun that comes from having 3 siblings close in age, but i'm also a tad jealous of the life she's had.
anyway, i'd say it can work, but it depends so much on the personality of the individual kids that there's no way to really plan on it. i didn't really like having a sibling close in age either, but that's because our personalities clashed... my youngest sister loves her life, but it's as much a result of her personality as it is of circumstance. i think you need to be mindful of treating your older ones as built in house help in any situation, not just when there's a younger sibling.