Mine are almost 6 yrs apart.
Initially, there is jealousy — but it gets better as they get older.
I'm hoping. My one brother and I were 5 years apart, and we fought like pitbulls between 1975 to 1981 (but granted, a lot of that also had to do with some seriously warped family dynamics.) We're now the best of friends: he's my A number 1 drinkin' buddy and gun guru (although, thankfully, not at the same time.)
My little one is really on my hubby and me now: "I want a little sister!" So I'll be 40 with the sleepless nights, etc -- but I'm a triathlete, I've got the energy now! ;-)