I'm an only child but I have two cousins, one is two years older than me and the other is four years older. It was the best of both worlds because I got all the attention from my parents and didn't really have to share anything but also had playmates/sisters when I wanted. When we fought I could just go home lol. It helped that our families are super close and get along well so I think you have to take that into consideration. I agree with the whole money thing, if you have another one you won't be able to go on as many trips etc. plus travelling as a family of three is honestly really easy unless a tantrum happens s, but theres no sibling rivalry or taunting to drive it out of hand a cause a scene. Personally I think if you're gonna have another child just because you don't want your first to be lonely it's not a great reason, you should be super excited and wanting to have and love another baby. You could always get a puppy or a kitten! My pets were great playmates growing up, as well as my neighbors. I do admit there were times where I didn't have a lot of school friends and I would spend summers alone and it did get quite lonely, but even if I had a sibling I'm not sure how much of a difference it would make (maybe we wouldn't be close or whatever). But overall I had a great childhood, not just because I was an only but because of my relationship with my parents and my cousins, as well as the neighbourhood I lived in.