Brothers & Sisters in iniquity

In literature, many instances of brothers & sisters instead of correcting, aiding each other in iniquity are observed, probably owing to their shared DNA.

In Virata Parva of Mahabharatha, Queen Sudeshna, wife of King Virata, instead of advising her brother Kichaka against his philandering, aids him & even puts pressure on Draupadi (who was serving her in disguise as Sairandhri) to accede to his wishes, resulting in the death of Kichaka.

In Tolstoy's "Anna Karenina", the adulterous behaviour of the title character, which led to her tragic death, was mirrored in the behaviour of her brother Prince Oblonsky, who himself was not a faithful husband, causing immeasurable problems to his  family.

In Jane Austen's "Mansfield Park", the siblings Henry & Mary Crawford, try to fool around the affections of the "made for each other" couple of Edmund Bertram & Fanny Price. Henry, true to his nature, elopes with a married woman. When Mary, defends him to Edmund, saying if Fanny had married him, he would not have done it, Edmund's eyes are fully opened to Mary's insidious behaviour, leading to his discarding her, & uniting with his true love Fanny.