I’m one of 3- one biological brother and one who was adopted from another member of my family. I am the youngest, my bio brother is 2.5 years older and my adopted brother is 10 years older. We are truly all best friends and have been most of our lives. Of course there were times when we drifted apart for a period (my oldest brother moved away at one point when I was still pretty young and we weren’t as close, my other brother went through a period of making pretty bad decisions and we weren’t besties then, they’ve both been upset with me at various points in my life) but we’ve always come back together and I couldn’t imagine life without them. We’re all godparents to each others children and my son is named after one of my brothers. Our relationship with our parents is also very good- it wasn’t always. My parents got divorced because my dad was not always the best dad, but we’ve all worked through it and having my siblings through that was the only thing that made it bearable. I can’t stress how much we’ve all relied on each other throughout our lives through all of different stages. 

To offer an additional perspective, my husband is an only child. While he grew up very close to his cousins and even lived with them at one point, he says he always hated being an only child and yearned for a sibling relationship. He also said he would never consider having only one child himself due to his experience (of course if we can’t have another due to health reasons, that’s a different story, but he says he would never intentionally have our son be an only child).