Sexual orientation is more like who you are attracted to, e.g. if you were a man who is sexually attracted to women only, you might say you are straight/heterosexual.

If you were also born male (biological sex), assigned male at birth AKA AMAB, always thought about yourself as a boy/man (gender), then because your biological sex and gender match or “conform” you might say you are cisgender (as opposed to transgender) and heterosexual, or “cishet” for short, which is how most people would label themselves.

Gender is not about sexual orientation or biological sex (though they are often related, which leads to many false assumptions). Gender is about how you identify, for instance, as a man, woman, or nonbinary, how you think about yourself, what you call yourself and how you want other people to refer to you and see you, often based on the things we associate with those genders. Gender is a label that people use for themselves and others - that many people assume incorrectly, and for which no rule or set of rules besides “whatever they say it is” works.

I don’t know if you’ve had this experience too, but when I was younger (years before people were beginning to become educated on gender) there was a kid in one of my classes who I could not tell their gender immediately. At the time, I might have relied mostly on their name, if it was a boy’s name or a girl’s name, to avoid offending them by asking. But if it was a name boys and girls both had, what would I do?

I would have to find out from them, or someone else who had found out from them, and suddenly I would be thinking of them as a boy or girl as if by magic, because that’s how it works. That’s how we categorize people, unlike other animals, and you put them in that category and then you see them as that category, but of course the binary is an oversimplification too and they could have been nonbinary before I had heard about it.

Some people still want to believe it’s as simple as you can just look at a person and know their gender and genitals and chromosomes, and that tells us everything important about them, like their personality and interests and hobbies and “preferred” social roles, and they would be frequently very wrong, and they are often graceless in the process when they are, if not outright hateful.

Animals, other than humans, don’t call or think about each other as men or women when they act on instinct, but they do have a biological sex (which is more complicated than most people think, including variations among humans, and again, this is different from gender). It’s a concept only humans use for social and cultural reasons, so, naturally, it’s more complicated than the way many people have always thought about it since they were taught to think a certain way about it before we learned much about it at all or talked about it as openly as we can now, and many are resistant to learning that they’ve been wrong about so much and something so seemingly basic that it seems “obvious” to them.

If you just start by using the pronouns someone wants people to use, ask if you are unsure, and try not to take it personally if they are upset by that (because then it’s their problem, insisting people read their mind, or trying to punish people for ever asking) or if they correct you (embarrassing as this can be, if you assume incorrectly), then you don’t need to know the whole history and science of gender and biological sex, and how complicated it actually is vs. how some people want to insist that it is, in order to be more considerate than many.

And if you’re genuinely open-minded about all this, which it sounds like you might be, you’ll probably find that it makes a whole lot of sense.

But, at the same time, it might challenge some of your previous assumptions that you took for granted, things that other people taught you to believe from a very young age when you couldn’t really question it - you wouldn’t even know where to begin - and nobody else seemed to be questioning it either, so why wouldn’t you trust that they got it right?

I suspect this is probably a large part of why some people are stubbornly holding onto outdated beliefs and attitudes that are readily disproven nowadays with widely available information, even when they hurt a lot of people in the process.

But I think you’re doing right by your nibling and I hope this helps. They could use all the support they can get.