Is it permissible for a Sunni man to marry a Shiite woman, and would the marriage be invalid if she does not follow the Sunni madhhab?

The Shi'a are a sect that differs from the Sunnis in a number of fundamentals and beliefs. Among their corrupt beliefs are reviling the Companions, slandering Aisha, invoking others besides Allah, believing in the distortion of the Qur'an, and that their saints know the unseen and are infallible. So, whoever holds any of these beliefs is not permissible to marry until she repents. If she does not hold such beliefs, then there is no harm in marrying her. However, if there is a fear of the influence of her family, it is preferable to avoid such a marriage. Since the girl is willing to abandon the Shi'a doctrine, this is an opportunity to clarify the truth to her, based on the Prophet's (peace be upon him) saying: "For Allah to guide one man through you is better for you than red camels," and it bodes well for the children in the future.