I have siblings and I tend to write books with siblings.
I have yet to write a story about an only child. The closest was my previous novel where my MC had a brother, but heās only ever mentionedāhe never appeared physically in the book.
I am actually curious though, and sometimes wish I was an only child.
My mom is an only child and the only things she really mentioned was how lonely it was unless she hung out with her cousins (who were close to her at the time). But I have four siblings (and a few others Iāve never metāa brother who died before I was born and a half-brother who probably doesnāt know I exist, from my dadās first marriage). But besides my siblings and parents, I donāt know anyone else outside of that to actually consider as close relatives.
With my mom being an only child, I donāt have any aunts or uncles or cousins from said people. She had all of that (my second, third, etc. relatives) but Iāve never met any of them, or if I did, it was when I was very young. And even then, most are dead by now.
And my dad was adopted, so we know nothing of his biological side. But from his adoptive side, he has a sister and Iāve only met her once. And I have cousins through her (her kids). Again, only ever met once.
So Iām boggled to even know of people, whether theyāre an only child or not, who have other relatives who are close to them. Iāve met people who consider their cousins, their siblings just because of how close they are. Like all I have are just my actual siblings. Lol
And like⦠how quiet is it? What do you do with your days? Is fighting and drama minimal, like to a point where itās peaceful? 
It is quite interesting lol. Even as someone with a large age-gap with one of my siblings, when I hear someone who has a 20-30 year age gap⦠like WHAT? 
My little sister is roughly ten years younger than me, and sheās fifteen years younger than my oldest sister (sort of first born). If my brother was alive, sheād be eighteen years younger than him. Thereās a few year age gap between me and my older siblings. The actual ages between all of us:
Cas: 31 years old (just passed her birthday).
Rach: 29 years old (turns 30 in November).
Bud: 27 years old (turns 28 in May).
Me: 25 years old (turns 26 in March).
Mel: 16 years old (turns 17 in August).
My deceased brother would be turning 35 in August, and I donāt know the birthday or age of my half-brother, but I know heās in his late 30s.
But like, Iāve known people whoāve said theyāre like a teenager and their sibling is in their 40s. Like how old were your parents when theyāve had yāall?
Truthfully, my little sister hates the fact that our parents are old (dad turns 60 in September⦠oof. And mom turns 55 next month) because they could die and not see her throughout half her adult life. 
I like writing sibling dynamics that reflect real life siblings (AKA, using my experiences as inspiration) but with a healthy relationship between them where theyāre actually close. In a previous story of mine, the focus was about these three sisters: Jane (26 years old, the oldest); Iris (24 years old, the middle child); and Kelly (21 years old, the youngest). I took inspiration of their dynamic based off my sisters and myself but with completely different stories and whatnot. Truthfully, Jane is a small reflection of my oldest sister where sheās very sarcastic and judgmental at times. In real life, my sister can be rude, make fun of me, and other things, but weāre actually very close. Kelly, on the other hand, is a reflection of my little sisterāvery bubbly, outgoing, and tries to be the peacemaker⦠even if we fight.
I think I write more healthy relationships between siblings just to show people they need to be nicer to their siblings. It surprises me that these days, thereās barely a sibling dynamic that is healthy and close. Every single one of my little sisterās friends who has siblings all hate each other for some reason. They all fight, canāt get along, and donāt want to be near each other. And they all envy us because weāre actually great with each other. Thereās times when we pull her out of school just to go to the movies or to get ice cream. Iāll take her to our local cafe in the mornings, or spoil her with gifts and treats. We hang out and watch movies or play games or do something with each other. Like, why arenāt there siblings who actually like each other?
Now, yes, sometimes siblings arenāt the greatest. Iāve cut my brother out of my life (for the most part) due to his toxicity. We were once close⦠as children. Growing up, my two older sisters were close with each other and not me. I was close with my brother until he went to middle school (became a teen) and after that, his whole demeanor changed. He became angry and abusive and acted like he didnāt care about anyone but himself. We stopped hanging out, and I tried to stay away from him until he moved out. Iām not the only one who cut ties either.
But this isnāt an every sibling situation thing.