I believe you are among...
the noble gases elemental gases
and this is my second third go, this time going line-by-line.
You asked me what I did before I fell asleep;
Perhaps you'll know if I tell you whose watch I need to keep.
Firstly, I believe you are the element hydrogen helium. As the first element of the final column of the periodic table, you "keep watch" over the elements below you. In another sense, blimps filled with He may watch the space below them.
I'm the freest of them all, all things at the least.
So carefree in the air, I shall wander as I please!
Helium is a light and plentiful element that is relatively difficult to contain. It is also the least reactive.
My elder brother's glowing hot, who's died in the colourful balls.
Your elder brother is helium neon. He "has died" because he is the a product of fusion within the "colourful balls," known to us as stars. There is a layer of neon within a typical star.
But it seems like he's enjoying it, so it ain't big matter at all.
It "ain't big matter" because the nucleus of He is relatively small. As a noble gas, Ne won’t react to much that’s going on.
My elder sister's lazier, sleeping within her glasses.
Your elder sister is argon. The name for Ar is derived from a Greek word meaning "lazy". This is because, as a noble gas, argon is not very reactive.
But she does witness the others burn, shining through her masses.
I learned from Wikipedia that argon is used in welding to shield materials from the atmosphere. Your sister would not be the one burning, but she would be there to witness it.
As for my second youngest brother, he's hiding in his room.
Your second youngest brother is krypton, whose name is derived from Greek for "hidden one". He must spend a lot of time in his room (or crypt, if you like).
His heart is constantly blue; his name hints a gloom.
Krypton can help incandescent lights appear brighter, which results in a more blue hue.
Regarding "gloom": The first three letters, when isolated, bear a phonetic resemblance to the word "cry."
Krypton is also the name of a fictional planet that blew up, which is...uh...depressing.
My younger sister follows, the naughtiest of us all.
Your younger sister is xenon. She is the naughtiest because she was the first noble gas that scientists were able to use to make a compound...
Lingering across the street, with her friends whom we appal!
...with Pt and F, who are "across the street" in different parts of the periodic table.
My youngest brother has died so young, we all felt sad for him.
Your youngest brother is radon.
At the age of 'bout a week, he left our family, so grim.
He died quickly because radon has a brief half-life and will thus decay somewhat quickly. (The half life of radon is 3.8 days).
Happiness follows soon, for we're expecting another sister!
Born to earth some time ago, she's just got on the register.
Congratulations on your newest member, Oganesson, who was "born to Earth" (synthesized) and eventually named in 2016! She is "expected" because her attributes can only be predicted at the moment; she could be a gas or even a solid.
So now tell me where standing I am,
And who is there in my large fam!
As explained above, you're helium, keeping watch over your Family, the noble gases.
