Uncanonical Relative

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 is also an uncanonical sibling.

One canon parent
If the mother of the Sue is mentioned she often has no more importance to the story than to give birth and die. Thus giving the Sue also the much needed angsty past.

On a few rare ocassions Sues have been encountered where the father was an OC. He impregnated the mother on a one-night-stand when she was a teenager, and was never seen since.

Two canon parents
There are two reasons why a Sue would be the daughter of two canon characters. The first is that she really wanted to be the full-sibling of a canon character. On a rare ocassion is the Sue the result of one of the canon pair cheating on the other half with another canon character.

The second reason is that she thinks two canon characters should be together and the height of their relationship is to have a baby. The fact that both characters are male does not pose a problem as this will naturally result in MPreg rather than adoption. All-male couples are more likely to have uncanonical sons than the average couple in fanfiction.

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 upto 20+ years.

If (one of) her parents is a 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.