"A brother and a sister."
Translation:Un hermano y una hermana.
What is the difference between un and una? Is un masculine and una feminine? And if that's the case, is there a nutre version of this word, does spanish have nutre words?
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