I'm 1nof 3, my dp is 1 of 3.
We have 3 children together and DP has an older son.
There is 2 years between my elder sisters and 15 months between middle sister and me.
I am close to middle sister but neither of us are close to elder sister however we come together at hard times, like when df died.
My parents managed it well, loft converted when we were early teens so everyome had their own room, enough money that we all had what we needed.
Dp is close to 1 sibling but not the other. Similar age difference between him and his siblings.
Our 3 kids are 8,6 and 5. We had planned on 2 as 3 makes an odd number, like when going to theme parks, eldest dd often ends up sat alone. Hotel rooms and family deals and life in general is built for 2 adults and 2 children.
However we are very glad for DC3 as DC2 is very disabled and as such DC3 and DC1 have each other to have a 'normal' sibling relationship with and although they fight they are incredibly close.
DSS visably hates being 1 of 4, though he was 10 when the eldest was born so was an only child for a lobg time and finds his siblings annoying.