"A brother and a sister."
Translation:Un hermano y una hermana.
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