Answer: These siblings are referring to the seven deadly sins.
Why: In the text nothing was said about the parents, so they had no relation to the answer. Originally there were eight deadly sins. They were gluttony, lust, greed, sorrow, wrath, sloth, vanity, and pride. The siblings go in this order:
Oldest Brother: Gluttony
Oldest Sister: Lust
Second Oldest Brother: Greed
Second Oldest Sister: Sorrow
Fourth Youngest Sibling: Wrath
Third Youngest Sibling (Sleepy): Sloth
Second Youngest Sibling (Sister): Vanity
Youngest Sibling: Pride
Later, vanity merged with pride, sorrow merged with sloth, and envy was added, forming the modern seven deadly sins. The siblings now using the modern seven deadly sins are:
Oldest Brother: Gluttony
Oldest Sister: Lust
Second Oldest Brother: Greed
Fourth Youngest Sibling: Wrath
Third Youngest Sibling (Sleepy): Sloth (+Sorrow)
Youngest Sibling: Pride (+Vanity)
Adopted Youngest Sibling: Envy
