A similar but different approach from my previous answer...thanks @ᴊᴀᴠʏ for the suggestion.
My body is pure,
surrounded by darkness
and crossed by it.
Pure as plasma! Fire is often surrounded by darkness, at least in comparison to its own emanation of light. "Crossed" can mean "marked" and the presence of fire is marked by darkness in the form of blackening of a wick, the ground, or whatever else it chars.
Opposite of my limiting siblings,
granting true freedom of choice.
Limiting siblings could ice, as in fire and ice. In many ways fire and ice is opposite - ice can restrict travel and is constriction of atoms, whereas fire emits light and heat and enables greater travel and choices. Our "inner fire" could be said to represent our freedom of choice as in will-power.
Ignoring me will do you no harm
but if you don't pay attention I may be the reason you die.
You don't need to pay much mind to fire, you can survive without giving it much thought as it won't come and bite you. However if you don't pay attention to hazards associated with it, it could be the reason you die (e.g. by house fire, burns, explosions).
The title also may hint to this answer as well!