Can a Shi’te twelver man marry a Sunni lady?

Shi'ism is one of the Muslim schools of jurisprudence. The difference between Sunnis and Shi'ites is limited to a few issues, the most important of which is that of the imamate [leadership]. Sunnis believe that the issue of the Muslims' leader [imam] is from among the ancillary branches of religion and that Muslims choose their leader through election and consultancy.

They also believe that, unlike Prophets, the leader is fallible. On the other hand, Shi'ites believe that the issue of imamate is from among the fundamentals of religion and that the imam is not chosen by the majority, his imamate is explicitly stated in legal texts [Qur`an], and he is infallible..
The Shi'a sect is divided into many extremist and moderate groups. The Twelver (Ithna 'Ashriya) and Zaydiya groups are an essential part of the Muslim community [umma] and the difference between them and between the Sunnis is of no significance. The madhabs [schools of jurisprudence] followed by these two groups are not only taught in the noble Azhar but are sometimes resorted to in issuing fatwas (religious edicts).

Based on this, it is permissible for this Shi'ite man to marry this Sunni girl if he is from among the moderate Shi'a groups referred to above. This is because both of them share the same beliefs even if they differ over some subsidiary issues and rulings upon which there are differences of scholarly opinions.

However, marriage between Sunnis and a Shi'ites is subject to environmental, social, and intellectual considerations that, in some cases, might not be compatible with those of others. We recommend that you take the above into account; you must also ascertain that the prospective spouse does not have strict ideas and opinions that insult the Companions or doubt the foundations of the religion.