"A brother and a sister."
Translation:Un hermano y una hermana.
You confuse "male" and "female" that are real genders, with "masculine" and "feminine" that are symbolic, grammatical, non real genders.
When you say "It's a girl", you talk about a female, and the female is also of feminine gender.
When you say "Es una silla", you talk about a chair, that's not a female (as it has no sex, and it's not living), but it's feminine, grammatically.
All nouns have genders in Spanish, even what we consider "genderless" things in English. Most of the time, masculine words end in -o and feminine words in -a. Take a quick read through this article for a longer description: https://studyspanish.com/grammar/lessons/genoun1
Yes, just like the sound in "feet".
Yep, masculine and feminine.
In general, there are no "neuter" nouns, but there are some nouns that are ambiguous in gender and can work with either article, as well as a limited set of other "neuter" words like "aquello", "ello", and "lo" that can be used to refer to unknown/unspecified objects.
Take a look at this article for more information: https://www.thoughtco.com/neither-masculine-nor-feminine-3078136
There are no neuters nouns, except neuter with the meaning of "indefinite or mixed gender", and it use the masculine appearance.
For instance, when you say "Los maestros", it can include maestros and maestras. It's not because only males count, it's because it's a collective neutral.
The noun that you are mentioned, are not ambiguous, they are either masculine (for instance un estudiante), or feminine (una estudiante), and they have a fixed gender.
The only thin is that they are "epicene" nouns, meaning that they have the same form in masculine or feminine, (the same "look"), but it doesn't mean they don't have a fixed gender.
Una estudiante is feminine, and un estudiante is masculine (it's only by "accident" that they have the same appearance.
So, nouns are either masculine or feminine grammatically, but can represent a collective neuter, or an indetermined neuter.
But pronouns can be neuter, as you said "lo", for instance can represent a verb (so it has no gender) or an action.
Sabes leer (you know how to read/you can read).
Lo que sabes = leer.
What you know = reading.
"Lo" represents the verb "leer", so it's genderless, as it's not a noun.
Yes, but not only "males". Males is only one of the subgroup that use "un".
"Un" is used for masculine. Masculine include males (men, boys, male animals...) and grammatically masculine innert things (un sombrero...).
"Una" is used for feminine. It includes females (women, girls, female animals...) and grammatically feminine innert things (la silla, etc...)