decadent children?

"In the presence of Cesare Borgia and his sisters, the lovely and vain Lucrezia -- decadent children of a wicked pope -- no one can claim innocence for very long."

Are "decadent children" illegitimate children or just bad children?
Besides, does "his sisters" refer to "the lovely and vain Lucrezia"? If it does, why is it in plural form? There seems to be only one sister in this sentence.

dec·a·dent (dĕk'ə-dənt, dĭ-kād'nt)
adj.
Being in a state of decline or decay.
Marked by or providing unrestrained gratification; self-indulgent.

Decadence has nothing to do with illegitimacy. It's more like being rich, spoiled, and jaded.

Re: "In the presence of Cesare Borgia and his sisters, the lovely and vain Lucrezia -- decadent children of a wicked pope -- no one can claim innocence for very long."

"Sisters" sounds like a typo. There was a Lucrezia Borgia (there is a famous painting of her stabbing herself that used to fascinate me as a child), so I am assuming that is the Lucrezia being referred to, and since she has the same last name as Cesare, she's probably his sister. They may have had other siblings as well, but if that is what the author is alluding to, it's not apparent from the choice of wording.