"Tomorrow we are going to go to my sister's house."
Translation:Mañana vamos a ir a la casa de mi hermana.
I brought this up in another thread, and have been told that "VAMOS A + INFINITIVE" is the usual way of stating the future tense in Latin American Spanish. The "INFINITIVE (as stem) + PERSONAL ENDINGS" form has an equal meaning, but is used less often.
In European Spanish, the "INFINITIVE (as stem) + PERSONAL ENDINGS" is the usual way, and "VAMOS A + INFINITIVE" has the specific meaning of being on your way to do something.
In both areas, you can use the present tense in the above sentence to translate into Spanish. So, in Spain, you could use 'Mañana vamos a la casa de mi hermana' or 'Mañana iremos a la casa de mi hermana'. In Latin America (but not in Spain) you can additionally use 'Mañana vamos a ir a la casa de mi hermana' to convey exactly the same meaning.
There are subtle differences in the feel of each of these, and as Oliwia said to know the best translation you need a geographical and conversational context.
Given all that's been said, and that this IS a lesson on the future tense, why aren't we practicing it more??? Almost all of the sentences have been the phrasal future of ir plus a plus infinitive, and I'm on the 5th of 6 lessons! Don't mean to be picky, but I know I need more practice with the other.
it conveys the same idea, but since the translation needs to be as closer as possible to the original sentence structure "Mañana vamos a ir a la casa de mi hermana" is the right choice. If you say "Mañana IREMOS a la casa de mi hermana" you are saying "we WILL GO tomorrow to my sister's house". I hope this helps.