Who is the painter known as theMaster of Sainte-Parente, and where is
the painting of Saint Sebastian he executed about 1500?

"Sainte-Parente" is the French phrase for the Holy Family; to make the
nickname consistent (without benefit of circumflex), it might also be
phrased as "le Maitre de Saint-Parente", "the Master of the Holy
Family", "der Meister der Heiligensippe", and other similar phrases.

The German version may point to the artist you're looking for; the
Catholic Encyclopedia's entry on religious painting
(http://www.newadvent.org/cathen/11395a.htm) reads in part, "At the
end of the fifteenth century there was no German town or province
which had not its local school. For a long time only two of these were
known or regarded: that of Cologne, with its anonymous masters, the
Master of the Passion of Lyversberg, the Master of the Death of Mary,
the Master of the Holy Family (Heiligensippe) [...]; and the school of
Nuremberg". So at the very least, someone in (late?) fifteenth-century
Cologne has the paradox of being anonymously renowned as "the Master
of the Holy Family", giving a close match for the name and time and
suggesting a place of origin.

Unfortunately, the iconographic bibliography for St. Sebastian at 
http://bode.diee.unica.it/~giua/SEBASTIAN/ , although conveniently
organized by the century of origin, fails to list this artist's
nickname. There's a more complete iconography at
http://www.sansebastiano.com/san%20sebastiano.htm , but it's based on
Italian artists and the database search is mainly written in Italian.
Exact location of this painting may require someone who can websurf in
German.

The painter you're referring to is probably "Maître de la Sainte
Parenté" - note the circumflex over the i  and the acute accent on the
final e, which are essential to do a proper search on Google. There
are a few images of his work at  
http://www.insecula.com/contact/A009950.html   Unfortunately, however,
like wombat138, I have been unable to trace any painting of St
Sebastian by him.

Maybe the painting is this one:
http://www.museenkoeln.de/wallraf-richartz-museum/default.asp?sam_01_00.asp~inhalt
http://216.239.37.104/translate_c?hl=pt-BR&ie=UTF-8&oe=UTF-8&langpair=de%7Cen&u=http://www.museenkoeln.de/wallraf-richartz-museum/default.asp%3Fsam_01_00.asp~inhalt&prev=/language_tools(page
translatoin)

And if that is the painting, it is in Wallraf Richartz Museum/Fondation Corboud
luntes