To an atheist faced with a dilemma of marriage
I would not be surprised that 90% of average Americans would answer a survey with, "Yes, I believe in God." Outside of science contacts (and this board), I don't know any American atheists.
Europe is another matter entirely, I do know several atheists
there who are not in the hard sciences.
I would be very surprised if that number applied to Europe.
I assume the comparison you meant was: Atheists help the poor more than religious people help the poor.
This would make sense, not because Christians are less decent, but because they trust God to deal with the poor.
Whether faith in God doesn't work, or faith in God is only taken up by those who are worse off to begin with, the result is the same. So you can't say for sure being Christian doesn't help.
Don't let your sister marry a Christian unless she understands how living with that will make her life different.
After that, it's up to her.