Learn Portuguese (Portugal): Confio na minha irmã.

In Portuguese, the verb confiar (to trust) is always followed by the preposition em (in). You don't just trust someone; you trust in someone. Here, em combines with the feminine word for "the" (a) to make na.

Why do we need the word for "the" (a) hidden inside na? Couldn't we just say em minha irmã?

In European Portuguese, you almost always put a definite article like a (the) before a possessive word like minha (my). So "my sister" is actually a minha irmã. When you add the preposition em (in) to the start of that phrase, they merge together into na minha irmã.

Where is the word for "I" in this sentence?

It is built right into the verb. Confio is the "I" form of confiar. Because the -o ending already tells us exactly who is doing the action, dropping the word eu (I) is extremely common and usually sounds much more natural.

How does the tilde (~) change the pronunciation of irmã?

The tilde over the "a" tells you to make a nasal sound, letting air flow partly through your nose as you say it. It makes the end of irmã sound somewhat like an English "uh" but with a nasal hum, similar to the French "un".