I thought “to” means THE or THAT. Shouldn’t this sentence mean: That is my sister?
Be careful here. Remember to distinguish between the demonstrative adjective "ten/ta/to/ty" etc. on the one hand, and the demonstrative pronoun "to" on the other.
It is the demonstrative adjective that is translated as either "the" or "that" rather than "this". This DL course is quite consistent on that point.
In contrast to the adjective, the pronoun "to" is invariable and is used for both singular and plural -- that is, for both "that is" and "those are". It can also be translated as "it is" (singular) or "they are" (plural).
But DL sometimes also translates the pronoun (not the adjective) "to" as "this" (singular) or "these" (plural). Perhaps this reflects a difference between English and Czech in how the pronoun is used.
For example, when introducing your sister standing next to you, in English you would always say "This is my sister", not "that". But perhaps in Czech you would say "To je moje sestra"?
Or would it be better to say "toto" or "tohle" when introducing her, and reserve "to" for pointing her out across the room?
"To je moje sestra." is more likely uttered when pointing at someone. When introducing someone next to jou "Toto je" or "Tohle je" is better.
In that case I think Paul378887 is right. For the main English translation here DL should use "that".
May be I was too quick with saying when you can use it. I would have to check the corpus, perhaps even the corpus of spoken Czech. It is for pointing, but it can be used for pointing to someone next to you too.
Yes, but when you revise the course (which is excellent, by the way) I think you should make "that" the main translation.