Different religion, does this one can be the reason not to get married?

Tweet

I have my sister who was having an affair with a guy who has having different religion from us..We are catholic and that guy was in other religion(i dont know exactly what was his religion), they are already having a son this time and until now they are not yet married coz of our religion. Coz if we ask to our parents they wanted that theire going to married in our church..but the guy side also is they wanted to married in their church..I am advising them to have a civil wedding coz i am thinking it was fine also...Was it civil wedding and church wedding are the same? what are the advantage and disadvantage of the two.....I need your explanations...:)