Transgender

Usually you use what they are now.

You use what they are now! If their past identity is important, then you can add "pre-transition" or something along those lines. For example, if you're talking about Elliot Page in Beyond: Two Souls, you say "Elliot before he transitioned".

You should basically never use a trans person's old name, gender, pronouns, etc. unless you absolutely have to. Treat them as if they've always been the person they are now.

There are of course some exceptions, like if they're not publicly out yet. You should never ever out someone else unless you have explicit permission. Or if there's just no other way to get the information across, e.g. if you're describing them to someone who only knows them pre-transition or if their legal name is required for something and they haven't changed it yet. If they explicitly told you they don't mind it's obviously okay too, but that's probably rare.

In short: Just keep in mind that bringing up their old identity is uncomfortable or even painful for many trans people and act accordingly.

They've also referred to themselves as the Wachowski siblings and the Wachowski sisters

I’m sorry but fake question. You know Lilly Wachowski transitioned. You don’t need to be told not to call her a man or “brother”. I’m all for making space for people to grow and learn, but if you know a man transitioned to a woman, then you know not to call that person “brother”. Like this is such a stupid fucking comment. My transphobic as all fuck mother would even know this.