Yeah, I agree with CraftMaker that it is preferable to have siblings. Being in a relationship with an only child (and being friends with many), I sometimes feel like they are not so great at discussing things/arguing.
I suspect this stems from not having had a sibling to “practise” arguing with. I know that might sound ridiculous, but if you think of only children’s experiences, it is either adults telling them what to do, adults nurturing them, or friendships where (whilst some arguments may happen) the intimacy and stability of a sibling relationship is not there, so there are far fewer arguments. I do think a lot of only children therefore don’t know how to argue as adults. My boyfriend has learned now (stick to the issue, don’t get personal, don’t lose your temper, stand your ground but try to see the other person’s viewpoint too), but I had to teach him how to do it, cos his first response to conflict was to just shut down. His mum and dad never argued with him, nor did his teachers or wider family. My friend, another only child, gets really cruel in arguments, turning it into a character assassination. So aye, I think missing out on healthy sibling conflict either makes you a pushover or else a bit of a “hothead” in later conflicts.
Having said all that, lol, there are various psychological studies about birth order etc, which say that it becomes irrelevant after a certain age (I think 10 or so, but you’d need to double check), cos the gap is too big. The eldest takes on a semi-caregiver role. So maybe the large gap will result in a totally different sibling dynamic than you anticipate.
One final thought (sorry! I’m very talkative tonight. I should not drink wine and then come on MN!). When my dad died (when I was 16), I really don’t know what I would have done without my older sister. Not that your Future death is a reason to have another child, lol, but I think that siblings are the only people who have always been there, who know your life experiences, your family dynamic - they just GET it.
I guess you need to decide if it’s a dealbreaker for you. Have you subtly tried to find out your existing child’s thoughts on having a sibling? Cos they might not even WANT one!
Anyhoo, good luck!