I have DSS, currently almost 19, DSD almost 16, DS 4 and DS2 on the way in summer.
DS has a 15 yr gap with his big brother and 12 with his big sister. He ADORES them and they dote on him, we dont all live together so cant comment on that.
There will be 4 yrs 4 months between DS and his younger brother, and they will live together so i am interested to know how this dynamic will pan out.
DSS now has his own DS, who is almost 1, so it changes things a bit, tho our DS loves it when DSS visits without the baby as he can climb all over him LOL
DSS and DSD used to fight like cat and dog but have always been close with a 3 year gap, apart from one 2 year period where DSS thought he was a big gangster and tried to tell DSD what to do etc. But i think thats personality rather than age!