All perfect praise be to Allah, The Lord of the Worlds, and peace and blessing be upon the Messenger of Allah, his family, and all his companions.
If your wife’s sister is an atheist, as you mentioned, and denies the existence of the Creator, then it is not permissible for a Muslim to marry her because Allah the Almighty says (what means): {And do not marry polytheistic women until they believe...} (Quran 2: 221). Therefore, your wife is not allowed to assist her sister in this marriage. Advise her in a kind, wise, and gentle manner. Assisting in such a matter is a sin but does not amount to Kufr (disbelief). From your previous questions, it is quite obvious that you suffer from OCD in matters related to disbelief and divorce and that you sent this question from a new email address. If that is the case, then we advise you to fear Allah the Almighty and then to not be carried away by obsessive thoughts of Kufr (disbelief).
Allah knows best.