Uncanonical Relative

An Uncanonical Daughter is a common type of Mary Sue.

She is the 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.