"My brother wants to go to the movies tomorrow."
Translation:Mi hermano quiere ir al cine mañana.
"Cine" is indeed "cinema", but when you say "go to the movies" in English, it's just an expression for "go to to the cinema". That expression, if you translate it literally to Spanish, would be "las películas" like you said, but it doesn't make much sense, simply because that's not an expression we use. So you must go with "ir al cine".
Maybe in Britain they would say something similar to "go to the cinema", since they use "films" instead of "movies", and "go to the films" doesn't sound quite familiar to me. Anyway, what matters is that "go to the movies" means "ir al cine" in Spanish. The literal translation "ir a las películas" doesn't make much sense.