“Give an extra piece of cake to a stepchild”
My informant heard of his Korean proverb from his grandfather but could never understand what it really meant. To him, he believes stepchildren usually have bad connotations tied to them, so he think it means that since the stepchild is either dumb or unpleasant to have around they should have one more piece of cake. They should receive it because the parent probably feels sorry for him/her being the stepchild and not blood related.
I believe the proverb is most likely saying something otherwise than what my informer claims. I’ve heard this proverb before somewhere and I think it is a proverb about parenting skills. Even though the child is not blood related, the parent or guardian should still provide them with the same love and care as they would if they were their own child.