Harry Potter twins dos and don'ts:

Making the twin the chosen one:

Do: make the twin a boy or have to go undercover as a boy.

Don't: make the twin obviously a girl when the prophecy says in the fanfiction that the chosen one is, in fact, a boy.

The twin's friends:

Do: Have the twin make friends with people from other houses— choosing characters that aren't in the spotlight all the time. When I made my Harry Potter twin sister book, still unpublished, she is good friends with Neville and the Weasley twins. These characters aren't being followed around all the time like Harry, Ron, and Hermione, and won't mess up the course of the original story.

Don't: Make the twin be best friends with the trio. Trio means three, not four. The twin can be best friends with either Ron or Hermione. Just not both.

Storyline:

Do: Have the twin live his/her own life away from Harry, not following Harry around all the time.

Don't: Have the twin do whatever Harry does whenever Harry does it. This makes the story uneventful.

I know many people make these fanfics and if you need any tips, since I'm not a pro, I just read too many fan fictions, these might be able to help you. Plz get to 1K, we're almost there!