Difference surname. 
Anyway, back to the topic.
Cousins right, as long as not close one (that means distant cousin
lah, like father's cousin's son/daughter or mother's cousin's
son/daughter), should be ok one. As for the close cousins (meaning
father's sibling's child or mother's sibling's child), because of
gentic issues, I have to raise a little objections.....
The same surname one hor, I'm ok with it one. Although the Chinese
view is those with same surname should not get married, thanks to
Neo-Confucianism, I see no reason to oppose it lor. Especially if
they are not blood related in any way.
From genetic point of view, I can understand why Neo-Confucianism
objects to marriage of same surname. However, since genetic doesn't
play a part in these marriage, I see no excuse to raise any
objections.