I’ve never been married, though you wouldn’t know

I’ve never been married, though you wouldn’t know.

I’m told I’m quite bright, but have no inner glow.

Seven siblings have I, at least that I know.

Some say I’m bloated, to others I’m fat.

But in reality it’s my brother that snacks.

What am I?

Perhaps you are

the planet Saturn.

I’ve never been married, though you wouldn’t know.

Saturn has rings. (But Saturn in Roman mythology was actually married...)

I’m told I’m quite bright, but have no inner glow.

A fairly bright planet, visible to the naked eye -- but Saturn, like all the other planets in the solar system, shines only by reflecting the sun's light.

Seven siblings have I, at least that I know.

The other planets. (Poor Pluto!)

Some say I’m bloated, to others I’m fat.

Saturn is a gas giant, next largest after Jupiter.

But in reality it’s my brother that snacks.

A reference to Mars bars (which I would never have thought of without seeing Foitn's answer).

• I'm not terribly convinced by the bloated/fat line, for what it's worth. (That plus the fact that I hadn't thought of the connection in the last line, found by Foitn, is why I didn't post this earlier.) – Gareth McCaughan Sep 19 '16 at 11:58
• Wow, that I didn't see that about the marriage.... Well done! – Foitn Sep 19 '16 at 11:58
• I'm just as horrified that I didn't get the last line :-). – Gareth McCaughan Sep 19 '16 at 12:00
• at least that I know is likely a reference to the newly-theorized "Planet Nine". – Charles Koppelman Sep 19 '16 at 16:09
• Jupiter's the biggest eater in the solar system. – Joshua Sep 19 '16 at 19:22

You are:

Jupiter

Because: I've never been married, though you wouldn't know:

This one I wouldn't know, sadly

I’m told I’m quite bright, but have no inner glow.

Jupiter is visible from earth and is quite bright, but doesn't produce that energy itself

Seven siblings have I, at least that I know.

There are 8 planets in our solar system

Some say I’m bloated, to others I’m fat.

It's the biggest planet in our solar system. Jupiter is filled with gas and is big. Unlike Earth, there is not any solid things like rocks on Jupiter.

But in reality it’s my brother that snacks.

Mars (another planets obviously) is a snack

• I've added a proper reasoning for "Some say I’m bloated, to others I’m fat.". – A J Sep 19 '16 at 11:47
• @AJ Your edit makes it a lot better, thank you, kind sir – Foitn Sep 19 '16 at 11:48
• @Foitn Nop but you are close. – user29679 Sep 19 '16 at 11:49
• Hmm. I had a closely related conjecture based on the "though you wouldn't know" but was unconvinced by the bloated/fat thing. rsp's comment makes me think it was probably right... – Gareth McCaughan Sep 19 '16 at 11:50
• Honestly, Jupiter was my first guess. In mythology, Jupiter (Zeus) was married but slept around with pretty much anyone and everyone. If you didn't already know he had a wife, you'd probably never guess it by the way he acted. Also, ancient astronomers used to think Saturn was eating its moons (or snacking on its children) before they realized that it was just the rings changing inclinations, alternating between the highly-visible side-on view and the practically-invisible edge-on view. – aleppke Sep 19 '16 at 22:23