When does kagome become a demon?

She didn't. At the end of episode 26 of Inuyasha: Kanketsuhen or Inuyasha: The Final Act (the last animated Inuyasha episodes), Kagome is shown as a human while Inuyasha remains a half-demon. It is strongly implied that they are already married, because Souta says his sister is to his classmates, and Kagome herself calls Sesshomaru her "older brother."

By this time, Sango and Miroku are already married (presumably have been for at least a few years, three at the most) and have three children: two girls and a boy. Kagome and Inuyasha are not shown with any children of their own.

Kagome turning half-demon at the conclusion of the series is what's known as "fanon," or fan-made facts about the canon that aren't true. It may also be the plot of a fanfiction, but it is not what actually happens in the show.