I think you are a
Nut - More specifically for most clues, a Peanut.
You can see me inside bars
Candy Bars or Pub-type bars(bowl of nuts/peanuts)
and also inside the stars.
oblate-spheriod stars (peanut stars)
Some may call me a key,
a nut on the end of a bolt, is sometimes referred to as a key.
The above line is a clue to thee!
rhyme time - (didn't see anything there)
My brothers are bigger than me,
sounds like a peanut, since a pea is very small and calling children "peanut."
I again say, the above line is a clue to thee!
rhyme time - (didn't see anything there)
I'm found inside a pack,
a pack of nuts or peanuts
Go ahead and try to crack!
a hard nut to crack - or cracking open a peanut shell to get to the nut.