#origamiwalrus

You’re not being rude at all, no worries. It’s good that you’re trying to be more respectful of other people!

I think the biggest problem with how your friend phrased that is outing their trans sister. It’s often dangerous to be known to be trans and you shouldn’t out someone without their permission. Some people might be fully out at school, but not work, out to friends and their sister but not their dad or their aunts and uncles, so it’s always better to ask rather than assume. “I have three little brothers and one little sister” would be better in most situations. Probably they told you that because you’re a trusted friend, which is great, but just as a general note :)

Slightly less worrisome is the “used to be my little brother” part. It’s maybe how you might explain it to a little kid - “she used to be my brother before she realized she’s a girl” - but sounds awkward to adults. When you talk about trans people, emphasize that they are what they have told you they are: a trans man “realized he’s a man,” not “became a man.” Gender can be somewhat fluid for some people, but he was probably always a guy, he just wasn’t out or hadn’t quite come to terms with it. Instead of “used to be a [gender]” just use “trans woman,” “trans man,” etc.

“Transsexual” is a really old word that used to be used for a trans person who underwent surgery/HRT. Although some people still use it, it’s rare now (and usually rude unless the person it refers to has specifically asked you to use it). Transgender or trans are better. Both adjectives, not nouns, so it’s “a transgender woman” “a trans student” not “a transgender.”

It’s good that you’re trying to become better informed, and this is a good place to do it! Let me know if you have any other questions :)