Straight and gay are words we use to communicate relevant information about ourselves and who we may possibly be interested in sexually interacting with, and who we definitely won't be touching ever. It is a factual, measurable reality of human experience.
"Cis" and "trans" are internal, so how does your brother know someone is "cis"? Maybe he's looking at someone who is xenogender? It's like speculating what shape soul someone has. www.lgbtqia.wiki/wiki/Xenogender
Xenogender is an umbrella term for non-binary gender identities that cannot be fully described through their relation to concepts typically used to describe gender such as male, female, woman, man, masculinity, femininity, androgyny, neutrality, agenrinity, or outherinity. Instead, xenogenders can best be described through how they relate to things, beings, or concepts that most individuals don't think of as having to do with gender, such as animals, plants, things, or concepts. It is sometimes described as any gender that "cannot be contained by sapi understandings of gender". The gender quality associated with xenogenders is xeninity. The opposite of xenogender is anthrogender.
Xenogender individuals often have a strong understanding of how their gender feels to them, however they often find that there aren't any words for their experiences, this is called a lexical gap. In order to fill that gap xenogender individuals often describe their gender through metaphors, by saying their gender is like, is influenced by, or shares qualities with something else.
When described, xenogenders often fall into one of three categories:
Nouns and Archetypes: instead of saying how one's gender relates to masculinity and femininity, one says one's gender is, or is like, a kind of animal, an imaginary being, a part of nature, an abstract concept, or a symbol. These are called noungenders.
Aesthetic and/or Synaesthetic Perceptions: when a gender is described using things like texture, size, shape, time, light, sound, or other sensory characteristics. These can often overlap with noungenders. These are called aesthetigenders.
Neurotypes: when a neurodivergent individual's neurotype(s) is/are an inseparable part of their gender identity. These genders are exclusive to individuals of the neurotype(s) associated with them. These are called neurogenders.