An uncanonical relative is a common type of Mary Sue.
This character claims to be related to a canon character, and thus worms her/his way into the story on the grounds that if their important family member is important, they must be, too. This is false. It can be assumed that most every canon character has parents (or a creator in the case of some nonorganism characters) and possibly other relations, and they may not be important or even mentioned.
You can expect an uncanonical relative to have powers similar to their canon relation(s), or perhaps the complete opposite if they are going for a yin/yang theme, but often end up more powerful than the canon character. Expect them to look very similar, too, often being "like x, but opposite gender" or "like x, but better/more skilled/prettier." Some Sues can accomplish this due to their Adaptable DNA, which allows their genetic code to change to match those of their chosen guardian figures.
It is not necessarily an indication of a Mary Sue that they are a relative of a canon character if the relationship is plausible and written logically, but Sues often use this quality to get closer to the canon characters without worrying about whether it makes sense or what impact their existence might have on their relative(s) beyond the story. Think of the Sues who are allowed to join the Fellowship of the Ring, Avengers, A-Team, etc., simply because they're the daughter or sibling of one of the team members, even if they're not properly qualified or even of age.
If the Sue is related to the canon characters in a way that could never happen (i.e., the daughter of two biologically incompatible characters or the living parent of a character with two confirmed dead parents), then this is a very important charge.
An uncanonical child is the daughter or son (but most likely daughter) of one or two canon character(s) that canonically don't have any children, or do have them, but not this particular one. In the latter case, she may also be an uncanonical sibling.
This child is often the Mary Sue of the fic they appear in, and as such is much more likely to be female rather than male. However, male slash couples (through MPreg) are more likely to have uncanoncial sons than the average pair.
Uncanonical Child of One Canon Character
In this variation, one parent is a canon character, and the other is an OC. In a goodfic, this may be liberating: no character should be forced to pair up with somebody from the main cast and shun all other options. Perhaps they met this person doing something offscreen, or their love interest simply hasn't been important to the canon story and the fanfic expands upon this behind-the-scenes character. In badfic, this is a cheap excuse to sneak an uncanonical child into the story, or worse, an uncanonical child and possibly a Mary Sue mother.
In badfic, if the mother of the child is an OC, she may be a Mary Sue herself. If she is killed off before the opening of the story, expect this dead mother to be the backbone of the child Sue's angsty past.
In badfic, if the father of the child is an OC, he may be a Gary Stu, but more often than not he is simply a Designated Abusive Bastard (to give the child somebody to angst about, rather than create a realistic drama) or an absentee father that impregnated the mother on a one-night-stand and has never been seen since.
A canon character father may also be hit with the Designated Bastard stick to give his Suvian child a convenient source of angst, even if he has to be driven OOC in the process. Alternatively, a canon character father who is normally evil or just not nice might be turned sweet and tender toward his precious daughter. (Gag us with a honey-spoon.)
Uncanonical Child of Two Canon Characters
In this variation, both parents are canon characters. In a goodfic, this may be the culmination of a long romance and a real commitment and even challenge to two characters that have been paired well and written competently. In badfic, this is a way to force two canon characters together, or even a ploy for cuteness and affection that ignores the gravity of parenthood as a life choice.
A badfic that creates an uncanoncial child between two canon characters may have ulterior motives:
- The author wants the child to be the sibling of a canon character.
- The author wants to create drama by creating a child as the result of one of the canon pair cheating on the other half with another canon character.
- The author may be using the child's ancestry as a tactic to produce an OC with the powers, abilities, skills, or influence of both parents ("because my father is a fire-bender and my mother is a water-bender, I can bend both fire and water/I can bend steam, which is a brand new element!") by way of bad biology. A tamer version might just want to make a cute tot with the physical traits of both parents, no matter how unlikely.
- The author thinks two canon characters should be together and the height of their relationship is to have a baby. Although many romantic parings have the potential to be executed well or at least sensibly, it is not true that having children somehow cements a relationship into unbreakability, or is able to redeem one or both of the parents into the perfect romantic couple. Relationships simply do not work that way, and many real-life stories of teen pregnancy often end in tragedy or despair at the hands of this immature fallacy. (Hint: if one partner has decided to get pregnant without the informed consent of the other, the relationship has gone very wrong.)
Furthermore, it does not pose a problem if both canon characters in question are male. Normally, one would assume that a homosexual pair in a slash story would simply adopt a child, but far more commonly slash pairs wind up involving MPreg instead. Like anything else, MPreg can be done well, but not usually when the above ulterior motives are in play.
The uncanonical sibling is usually is the twin or younger sister of a canon character. Older sisters and any sort of brother are extremely rare. Fandoms that are particularly plagued by uncanonical siblings tend to be fandoms where the main characters are in the same age group as the target audience. Although there are also cases where the age difference between the siblings can be up to 20+ years. Loud House badfics often add an eleventh sister or second brother to the Loud family.
If at least one of her parents is an established canon character, she is also an uncanonical daughter.
The uncanonical parent is rare, but they do exist, usually as an antagonist. Often this character is the "secret parent" or the "real parent" of a canon character and is only revealed to be related later in the story. It is possible these characters are inspired by Darth Vader from Star Wars—often they are primary or secondary antagonists, and almost always they are redeemed at the end of the Suefic by the power of Twu Wuv.