Learn Italian: Sei la sorella o la madre? - Are you the sister or the mother?

In Italian, the subject pronoun (tu for "you") is usually dropped because the verb itself already tells us who the subject is. Sei means "you are". Also, notice that you don't need to change the word order to make a question like you do in English. You just use a questioning tone of voice!

Is o the only way to say "or" in Italian?

O is the most common word for "or". You might also come across oppure, which means the exact same thing but adds a little more emphasis. Also, unlike the word for "and" (e), which sometimes adds a "d" before vowels (ed), o almost always stays as o.

I learned that we shouldn't use la before family members. Why is it used here?

Great catch! You drop the definite article (il or la) before singular family members only when you use a possessive word like "my" or "your" (for example: mia sorella means "my sister"). But here, we are just saying "the sister" and "the mother", so the regular articles are required.

Could I say mamma instead of madre?

Yes! Just like in English, madre is the standard word for "mother", while mamma is the more affectionate and common word for "mom". Both work perfectly in this kind of sentence.