The Closer I get to you...

How close are your kids?  Sometimes I wonder if the relationship between my kids would be considered "close"? I mean the youngest (4) seems to be close to everyone but the relationship between my oldest (13) and middle two (7&8) just doesn't seem close enough, or at all for that matter! I see other families and the kids all help each other and get along so great but then I look at mine and see almost a disconnect? More of a battle to see who can get more of my attention then actually having fun TOGETHER!! I understand sibling rivalry believe me I'm the baby of the family.  I wonder if I didn't or haven't done enough to try and help them be closer?