I have on younger sister (by 2 years) and a younger brother (by 3 years). All fairly close in age.
As kids, my sister and I were the closest and my brother was the odd one out - a lot more immature and the "golden boy" who was smiled on by everyone (in our extended family, I really can't fault my parents), so it took him a lot longer to grow up... He was always trying to curry favour with the adults by telling tales whenever possible. I bought him a Smirnoff ice for his birthday when he was 15 or 16 (just one, and a one-off, as well as a proper present - cos 18yo me thought it would be cool to treat him like a grownup as most of his friends' older siblings would no doubt be doing the same) and he told mum on me!
As teens/early 20s my sister and I fought like cats and dogs. We knew all the right places to push each others' buttons. It didn't help that we have very different personalities; she's inherited a particular prickliness and perfectionism from my dad's side and my mum's shyness/awkwardness, whereas me and my brother are similarly laid back as are most of mum's family and a bit more chatty to strangers, like my dad. And looking back I think a fair bit of that was jealousy when I went off to Uni and came back on weekends wanting to spend that time with my friends and my boyfriend, and I didn't get why she had to spend every weekend sniping at me, slagging my friends and calling me a slut... It didn't make me want to take her out with us at all, poor thing.
In our mid 20s we all became good friends. Just in time... In the last few years, my dad has been diagnosed with cancer (twice), my parents have had a lot of money worries, and my dad's family has imploded.... again. There are seven siblings and they are forever feuding, and seeing the fallout to our parents (typically, they tend to "forgive" my dad post-fallout and then resent my mum for not siding with them against her husband, even when she thinks he has done nothing wrong) has made the three of us more united and determined never to let that happen to us.
We don't see each other regularly as we live in different cities, my sis is two hours away and my bro is in Dublin. But we Skype now and again and will stay on for hours. My sister comes to visit me every six weeks or so, when she's not working weekends, and she has her own key to my place. And they treat my DH like an older brother. It's nice, I feel like one of a team of four.