My daughters are 34 & 37. The sibling rivalry settled down when the first one got her driver’s license. They are completely different Humans. But they respect and love each other.
Sisters will bicker — BUT — my girls will close ranks and maim anyone who hurts THEIR sister. I’ve seen it done.
I think the missing piece in your life were parents who knew how to parent. The balance. I was an only child and I HATED it. I had no one to play with and I grew up to be an awkward child with adult social skills in a kid’s world. I didn’t know how to play. My parents were 43 & 50 when I was born as well.
You were a great Father/Mother to your kiddo. It wouldn’t have matter how many kids you had — they would have been fine. Because Love.
Love is the key ingredient. And you have heaps of that