Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh
In the Name of Allah, the All-Merciful, the Mercy-Giving
All praise is due to Allah. Peace and blessings be upon His Messenger Muhammad.
I am not in favor marrying across sects for the much difference in faith and cultural practices. If you add to that how your family looks at it, it should give you another hint.
I am not going to list the differences between Shia and Sunni here, you can read about that but I feel the spiritual harmony as believing in the same tenants is very important for a long lasting and trouble-free relationship.
I have observations also on the social network matching sites as to what really I the character of the person. People tend to type the best of practices but not until you know their family, friends and character is when you might have some sort of idea of how really people are.
In short, I will advise of a better method to know people and someone from you own sect.
Almighty Allah knows best.
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