Wa `alaykum as-salamu wa rahmatullahi wa barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
1- It is allowed for a Sunni Muslim to marry a Shiite one as long as he or she believes in the basics of Islam, doesn’t violate the ethics and values of Islam, and respects the Prophet’s family and Companions.
2- However, a Sunni Muslim is best advised to seek a spouse of the same sect as marriage is a permanent bound. So, it would likely for spouses to lead a happy peaceful life if they share the same thoughts and beliefs.
In his response to your question, Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
Intermarriage between sunnis and shi`as generally is allowed. But mind that it is most desirable to make a pre-nuptial contract in order to rule out any possible conflicts due to differences in beliefs and practices.
However, marriage is a long term commitment and partnership. Since there are differences between certain beliefs and practices of Sunni Muslims (ahl al-sunnah wa al-jama`ah) and the Shiite (Shi`ah), you should only be willing to tie the knot after you have figured out how to deal with the differences in an amicable way.
It is not realistic to say that we would sort it out later.
So, I urge you to list all the points at stake and come up with a balanced and amicable way of dealing with the differences.
You should stipulate that both of you should be able to follow your respective madhhabs without sacrificing the marriage relationship.
Once you have agreed upon the salient points, you should sign a contract to that effect. There is a great word of wisdom saying, “There is no intelligence greater than planning.”
Allah Almighty knows best.