Answer:
I can be as thin as a picture frame but my insides have many things you can see.
One am I, among six others: Largest, smallest, cold, dark and two brothers
Blind but cruel.
What do you call the motherยญ-in-ยญlaw of your sister's husband?
An utensil used for bread. Also a paper cutter. Used by a thug to take a life, Or wielded by the tamest wife, When used to spread the butter.
Though not a plant, has leaves. Though not a beast, has spine. Though many wouldn't need this thing, It's more valuable than wine.