My First Guess is:
Mercury
I have a daunting mom and seven children
Mother = Sun, 7 children are the other 7 planets that come after mercury (excluding Pluto)
I outshine my sister
Closest to the sun
but my niece is quite famous:
Earth (not quite sure how it relates to niece)
Despite my big family, it's a boy with no parents, That made me a symbol of a freezing expanse.
I was thinking Mercury thermometer which was invented by Daniel Gabriel Fahrenheit
My second guess is:
Earth
I have a daunting mom and seven children
Mother = Sun, 7 children are the other 7 planets that come after mercury (excluding Pluto)
I outshine my sister
Sister is Venus
but my niece is quite famous:
(not quite sure how it relates to niece)
Despite my big family, it's a boy with no parents,
Aristotle
That made me a symbol of a freezing expanse.
Earth as a classical element represented solid - ice is the solid form of water