That which I am 2 - who are we?

Question

Who am I? I am many things.

I am the moon.

I am a physician, purifying the waters about me.

I am the bane of evil in the depths of the night.

I am the gateway to a realm contrary, though none may enter it through me.

I am the watchful record of the past.

I am a rain-bringer.

I carry the lightning, though people tend to prefer to ask my sister to do so. Well, that's fair; she is cheaper to hire.

Oh, no, I'm not an only child. I have two siblings.

My younger sister came from the Mediterranean, and it shows. She has a wonderful tan, though sometimes she looks a little green around the gills.

My elder brother, on the other hand, is favored by all. This, despite the fact he can't even say his first name, at least not without stuttering.

I also have many cousins.
Two, in particular, might be mistaken for me, at least for a moment.

The first of these - well, he has a hellish stench. And he likes to tarnish my reputation. Not fun. Still, we just have sound-alike names. He doesn't pretend to be me.

No, that's left to the other one. She looks a lot like me, I'll admit, although she's much more athletic.
And she can make herself useful at times, but be warned; don't get involved with her. She's pure poison, after all.

So, now that I've told you about myself and my family, do you think you can tell me who we really are?

Who am I? Who are my siblings? And who are my cousins?

Answer 1

You are

silver

I am the moon.

Silver is traditionally associated with the moon.

I am a physician, purifying the waters about me.

Silver is used to disinfect water.

I am the bane of evil in the depths of the night.

Silver bullets are supposed to kill werewolves.

I am the gateway to a realm contrary, though none may enter it through me.

Mirrors are made with the process of silvering.

I am the watchful record of the past.

Gramophone records are called "silver platters".

I am a rain-bringer.

Silver iodide is used for cloud seeding.

I carry the lightning, though people tend to prefer to ask my sister to do so. Well, that's fair; she is cheaper to hire.

Silver is a good conductor of electricity, but typically copper is used instead.

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Your siblings are

copper and gold

My younger sister came from the Mediterranean, and it shows. She has a wonderful tan, though sometimes she looks a little green around the gills.

Copper is named after the Mediterrean island of Cyprus. It has a red-orange color but can develop a green patina.

My elder brother, on the other hand, is favored by all. This, despite the fact he can't even say his first name, at least not without stuttering.

Gold is used to denote 1st place. The Latin word for gold (aurum) is derived from a reduplicated Proto-Indo-European root *h₂é-h₂us-o-.

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The cousins mentioned are

sulfur and mercury

The first of these - well, he has a hellish stench. And he likes to tarnish my reputation. Not fun. Still, we just have sound-alike names. He doesn't pretend to be me.

Sulfur (alternative name brimstone) is associated with Hell. It is known for the bad odor of its compounds, particularly hydrogen sulfide. Tarnished silver develops a coating of silver sulfide.

No, that's left to the other one. She looks a lot like me, I'll admit, although she's much more athletic.
And she can make herself useful at times, but be warned; don't get involved with her. She's pure poison, after all.

Mercury is also called quicksilver and has a similar appearance to silver, but it is liquid. Mercury and its compounds are toxic.