I am the oldest of 6 ( although 2 are step sisters and have only recently been back in contact with after 12 years apart, so I will exclude them in this response).
Between me and my sisters and brother there is 10 years, 12 years and 15 years difference. Tbh I hate it. There is 4 years difference between my youngest sibling and my oldest daughter. The same age gap between DH and his younger siblings. We always agreed that we didn't want a massive gap between our children. There is 3 years between DD1 and DD2, and even though DD2 is only 18 months, they are best friends. They are always together. They rarely argue. If one is not at home (like when DD1 goes to daycare) the other walks around moping until they are together again. There has never been any jealousy between them.
Having said all that, we didn't expect their relationship to be so good. We didn't expect them to be best friends. we didn't go into the second pregnancy thinking "oh yay, now DD1 will have a best friend. They will never fight. They will always be with each other". It just happened to go like that. Which I am very grateful for.
Me + He = DD1 (2007), DD2 (2010) & BellyBaby due August 2012
Angel Baby 1 (July 2008) & Angel Baby 2 (October 2011)