I don't know, I think that you can give a character a sibling if it's known about them that they are/were likely to. Hermione was supposed to have a little sister, I know that JKR accidently didn't write her in soon enough, but I think that it's fine to write her in herself as long as she played a role that seemed like what her canon one would have been. Obviously we wouldn't like upstaging or Mary Sues with this.
As with Ron and his family- he was supposed to have a cousin, who turned into that awful reporter. I could give him a cousin, but as above, not too much emphasis or screentime, they'd be extras, something to spice up the story and make it seem a little different than others.
I think that a story with Sirius' brother, Regulus never killed would be cool, but only if the writer were talented and did it RIGHT.
For the most part, what you say is true for the writers who can't do characters well, especially semi-original ones, but I think that the writers of good caliber who can handle these things with caliber are exempted.
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As with Ron and his family- he was supposed to have a cousin, who turned into that awful reporter. I could give him a cousin, but as above, not too much emphasis or screentime, they'd be extras, something to spice up the story and make it seem a little different than others.
I think that a story with Sirius' brother, Regulus never killed would be cool, but only if the writer were talented and did it RIGHT.
For the most part, what you say is true for the writers who can't do characters well, especially semi-original ones, but I think that the writers of good caliber who can handle these things with caliber are exempted.