So You Really Wanna Write An OC Relative of a Canon Character?

Being related to a canon character is one of the hallmarks of a Mary Sue. That said, it doesn't make a character a Mary Sue. It's only reasonable that most characters should have family somewhere, and that many of them will have children eventually.

So, here is a list of tips to create tolerable, if not even likeable OC relatives, who will be less likely to shatter a fanfiction or roleplay than otherwise.

First, avoid wonderbabies like the plague. They're hated even when the original authors write them.
Get at basic grasp of what types of traits are dominant and recessive, and how dominant/recessive genes work. If Mommy has red hair and Daddy's a blond, baby isn't going to grow up with raven locks without a dye job.
Don't create characters who are the spitting image of the canon relatives, or look like genderswapped versions of their canon relatives.
Do not make their personalities and tastes essentially the same, either. This goes especially for child characters in their teens and beyond - how many people do you know who act like or emulate their parents that much by the time they're your character's age?
Give your characters lives that don't revolve around or completely relate to the canon relatives. Give them friends, hobbies, interests, likes, life goals etc. that the canon relatives do not share nor have any interest in.
Don't have your characters become bestest buds/teammates FOREVARZ with their canon relatives. Nothing wrong with a friendly relationship, but remember - your characters' lives should not revolve around or always relate to the canon characters. Do you always hang out with your parents and your parents' friends and/or co-workers? Probably not.
Don't create characters who have more or less the exact same skillset or abilities as their canon relatives. Give your characters their own unique strengths and weaknesses.
Actually research the backgrounds of the canon characters. Don't be the next doofus to give Captain America a sister living in the early 21st century who (barring time-travel shenanigans) isn't well into her sixties at the very least.
Don't rewrite what has already been established in canon. Your character was not in the van with Jane Foster when Thor crashed to Earth. Your character was not on the raft with Will Turner. Your character did not tag along and help Sherlock Holmes solve his mysteries. Your character was not hiding behind the sofa when Voldemort tried to kill Harry Potter.
Avoid creating characters who require convoluted and/or extraordinary explanations just to fit into the universe, IE, time-travel shenanigans, being kidnapped as an infant, or having a parent who is a member of an alien race that doesn't get out much or that no-one's ever heard of before in canon.
Avoid creating long-estranged relatives, especially ones who just happen to show up when your story starts and insert themselves into the daily lives of their canon relatives.
During and after creating your character's past, put yourself into the shoes of characters your character would have grown up around and ask yourself how would they have interacted with your character throughout xir life. This doesn't just go for the characters you want your character to be friends or enemies with - this goes for everyone your character would have interacted with up until the point the story/roleplay begins.
Put yourself into the shoes of your character's canon relatives, especially any canon parents/caretakers. Ask yourself how you'd feel and react if this was your child/ward/whatever doing whatever it is your OC is doing.

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