I'm still sorting out what we really mean when we keep referring to the guy as "an uncle". Is he also a father? Is he married? What's the description of "uncle"

I'm still sorting out what we really mean when we keep referring to the guy as "an uncle". Is he also a father? Is he married? What's the description of "uncle" adding or informing us of? My immediate reaction makes me think he's a 50yo with no kids, maybe a girlfriend, but his sister had kids. I don't know that this is an accurate perception or if it even aligns with how other people read it. Just seems kind of weird that he's summarized as this title.