http://www.understanding-islam.com/rsi/si-004.htm
An answer to your question needs to be given from two different perspectives: the legal perspective and the personal or the individual’s perspective.
As far as the legal perspective is concerned, marriage between two Adult Muslims cannot be hindered. No one has the legal right to hinder this marriage. Shiite and the Sunni schools are both legally recognized schools among Muslims. None of these two sects has been legally declared to be non-Muslim by the Muslim Ummah. Thus, marriage between individuals ascribing to these two sects actually implies marriage between two Muslims, which obviously cannot be hindered.
As far as the personal or the individual’s perspective is concerned, it is important to realize the socio-moral role that our family, our friends and our society plays in our lives. Although our family and our friends and peers do not have a legal authority to hinder our marriage and other personal decisions, yet they do, to a great extent, influence us in all our social decisions. In this situation, I would recommend that the family and the peers be convinced of the legal and moral rights of the individual in question and asked to accept the individual’s decisions. I think it would only be reasonable on the part of all family members to finally give in to the individual’s decision, as this is ultimately the individual’s right to make the decision. On the other hand, the individual be fully informed of the differences that exist between the followers of the two sects. Marriage is a life long commitment. Therefore, decisions regarding marriage should not be taken in haste. We must fully understand the beliefs and practices of the person with whom we plan to get married. If after our knowledge of the other person’s beliefs and practices, we are of the opinion that the differences are quite trivial and should not hinder us from marrying the individual, we may decide to marry that individual. On the contrary, if we believe the differences are quite significant, then we should avoid risking our own and the other person’s future by taking an emotional decision.
To summarize, in my opinion:
1. The referred marriage cannot be legally hindered.
2. Family and peers have a right to pursue an individual to take a decision, which in their opinion is more suited, keeping in mind that it is ultimately the individual’s prerogative to take the decision.
3. The individual should take the decision after a thorough understanding of the differences that exist between the two schools. Emotional decisions taken in haste and in ignorance may cost us our future happiness.
I hope this helps. In case any aspect of my answer remains unclear, please feel free in writing back to me at your own convenience.
May the Almighty guide us all to the path of His liking.
Regards
The Learner
They shoot partypoopers, don’t they?