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 assumed that most every canon character has parents or relations (or a creator in the case of some nonorganism characters) and they may not be important or even mentioned.
You can expect them to have powers similar to the canon character, 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.
It is not an indication of a Mary Sue that they are a relative of a canon character, but Sues often use this quality to get closer to the canon characters: often being "like x, but opposite gender" or "like x, but better/more skilled/prettier."
If the Sue is related to the canon characters in a way that could never happen (IE, 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.
Uncanonical Child
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 it appears 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 and One Original 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 a possible Mary Sue mother. In badfic, if the mother of the child is an OC, she may be a Mary Sue herself, giving birth to Mary Sue children. 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 an abusive father (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.
Uncanoncial Child of Two Canon Characters
In this other 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 full-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 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 all 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 ('If my boyfriend concieves a child with me, he HAS to stay with/marry/love me!') often end in tragedy or despair at the hands of this immature fallacy.
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.
Uncanonical Sibling
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 are fandoms where the main characters are in the same age group as the author. Although there are also cases where the age difference between the siblings can be up to 20+ years.
If (one of) her parents is an established canon character she is also an uncanonical daughter.
Uncanonical Parent
The uncanonical parent is rare. But he/she does exist, usually as an antagonist. Often this character is the 'secret parent' or the 'real parent' of a canon character and is only revealed as 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.