No nos sentamos a la mesa hasta que mi hermana termine el puré.

Because the verb here is sentarse, which means to sit down. It is a pronominal/reflexive verb, so it needs a reflexive pronoun.

• me siento = I sit down
• te sientas = you sit down
• se sienta = he/she sits down
• nos sentamos = we sit down

Literally, nos sentamos is something like we seat ourselves, but in natural English it just means we sit down.

Without nos, sentamos would come from sentar, which usually means to seat someone or to make someone sit down.

The form sentamos can be either:

• present: we sit down
• preterite: we sat down

In this sentence, it is present tense, but it is being used with a future meaning:

• No nos sentamos a la mesa hasta que...
• We’re not sitting down at the table until...
• We won’t sit down at the table until...

This is very common in everyday Spanish.

Because sentarse a la mesa is the normal Spanish expression for to sit down at the table.

• a la mesa = at the table / to the table
• en la mesa would usually suggest on the table

So sentarse en la mesa can sound like someone is literally sitting on top of the table.

Hasta que means until when it is followed by a clause with a verb.

Here we have:

• hasta que mi hermana termine el puré
• until my sister finishes the purée

Compare:

• hasta mañana = until tomorrow
• hasta las cinco = until five
• hasta que llegue = until he/she arrives

So:

• hasta
  • noun/time expression
• hasta que
  • verb clause

Because after hasta que, Spanish uses the subjunctive when the action has not happened yet and is still in the future.

So:

• hasta que mi hermana termine el puré = until my sister finishes the purée

The sister has not finished yet, so Spanish uses termine rather than termina.

Not really. In this sentence, the subjunctive is not mainly about doubt. It is used because the action is pending or future from the speaker’s point of view.

So termine does not mean the speaker is unsure whether the sister will finish. It just means the finishing has not happened yet.

This is the standard way to say not... until... in Spanish.

• No nos sentamos a la mesa hasta que mi hermana termine el puré.
• We don’t sit down at the table until my sister finishes the purée.
• We won’t sit down at the table until my sister finishes the purée.

The no tells you that the action in the main clause does not happen before the action in the hasta que clause.

Spanish often uses the definite article where English might leave it out.

So terminar el puré is a very natural way to say finish the purée or finish the mashed potatoes.

English is often more flexible about articles here, but Spanish commonly keeps el.

It means to finish the purée, but the exact nuance depends on context.

It could mean:

• finish eating the purée
• finish making the purée
• finish serving the purée

In a family mealtime context, many learners would understand it as finish the purée in the sense that she is not done with it yet. Spanish often leaves that detail to context.

Yes. That is also correct:

• No nos sentaremos a la mesa hasta que mi hermana termine el puré.

This is a more explicit future tense version of the same idea.

Difference in feel:

• No nos sentamos... = very natural, conversational
• No nos sentaremos... = more clearly marked as future

Both are correct.

Because with hasta que, Spanish normally uses the indicative only when the action is viewed as habitual, completed, or already factual in context.

Here the finishing is still in the future, so Spanish uses the subjunctive:

• hasta que mi hermana termine

If it were a habitual statement, the indicative could appear in other contexts, but not in this future-looking meaning.

Usually sentarse emphasizes sitting down, the action of taking a seat.

A helpful contrast is:

• sentarse = to sit down
• estar sentado/a = to be sitting / to be seated

So in this sentence, nos sentamos a la mesa focuses on the action of taking our seats at the table, not just already being seated.