Who is Germany little sister in Phineas and Ferb?

Germany doesn't have a little sister in Phineas and Ferb. There are many countries personified as characters in the show, but Germany isn't one of them.

The characters that represent countries in the show are mainly:

* Canada: A laid-back, friendly, and somewhat awkward country with a love for hockey.

* United Kingdom: A somewhat snooty, but ultimately well-meaning country with a love for tea.

* France: A stylish and romantic country, often associated with cheese.

* Japan: A technologically advanced and sometimes shy country.

If you are thinking of a different character, please provide more details so I can help you!