I know everyone has their own interpretations of angels but in terms of Supernatural, are all angels actually related or do they call each other brothers and sisters for another reason? I'm sorry if this question is stupid.

It's definitely not a stupid question !

They aren't literally siblings, because they weren't literally born, they were created. They all have the same creator, who is to them their father, since angels do not physically breed. Although this doesn't mean he's not "really" their father, because this sort of father-child/creator-created relationship is much more powerful and real to angels probably than how some humans consider God to be their "father." Angels exist in a spiritual way, and a spiritual connection is probably more important than a genetic one. I don't think the angels even have any genetic material, until they possess a human vessel, they exist on a spiritual plane. In this sort of situation the concept of what is a parent or sibling becomes much more abstract, and whether angels are actually siblings depends on how literal your definition of the word is. However I think the angels consider eachother to be their brothers and sisters.

That's my take on it.

I think its like a respect thing.

What that means is as christians & followers of christ we are all related for God is are creater he is are father in heaven so that makes all of us brothers & sisters in Christ. Talk about your Big families!
Hope that didnt confuse you to much,

They're actually related -- although there are so many of them that I doubt they've all met each other .. they ARE all brother & sister. :)