While I don't have experience with being a *mom* to siblings who are spaced far apart, I do have experience with being a sibling in that circumstance. I come from a huge family, and my oldest sibllings are both 14 years older than me. Seven of my siblings are AT LEAST six years older.
The age difference isn't really an issue. It was probably one of the reasons we did get along so well. There isn't the competition like there is with kids closer in age. We didn't fight for the same toys, wear each others clothes, or need the same kind of attention from our parents.
My siblings were older and more independent, so they were able to provide my parents with more help around the house, making it easier for them. They were able to help babysit as we grew older. They were older and wiser, and acted like role models and advisors for us younger kids. (I even remember my parents using the car for leverage. "You can borrow my car, if you bring your little brother and sister with you." LOL!) They would take us to amusement parks, and things that my parents didn't care for. They would take us to concerts when we were tweens. (My first concert was a Bob Dylan concert with my brother seven years my senior. We still share a love of Dylan's music.) They even helped teach us to drive.
My siblings are all grown with families of their own now, and live all over the country. But we are still very close, and get along great. I look up to all of them, and appreciate the experiences they shared with me, the mentoring they provided, and the wisdom they shared. They are very protective of us, and value the experience we gave them with children, that they are appreciating now that they are parents.
Don't fret about age differences. It's not the closeness in age that makes the difference. It's the familiy atmosphere, the love, the time shared and the experiences shared, that makes all the difference.