Does one sin who says to his sister, "It is my duty to play dumb with you," while not intending to introduce tomfoolery into the religion?

Useless speech is prohibited because it can lead to something disliked (makrooh) or unlawful (haraam). The sister's statement to her sibling is frowned upon by people, even if it is not haraam, especially if it is meant as a joke. A piece of advice for the speaker and her sister is to occupy themselves with the remembrance of Allah and the recitation of the Qur'an, to maintain good morals, and to abandon what is disgraceful.