Hi, my brother is getting married but not in the church.
You do not say if both parties are Catholic, whether he is practicing his faith currently, or whether your brother has had an appointment with his pastor to go over what must be done for a valid Catholic marriage.
So, what you really need to do is encourage your brother to make an appointment with his pastor to discuss his impending marriage.
I need to know if he would be living in sin? Is that mortal sin?
Catholics are required to follow the marriage laws of the Church. If they do not do so, that is grave matter against the sixth commandment and the precepts of the Church.
It is possible for a Catholic to marry a non-Catholic in a non-Catholic ceremony if the Catholic follows all the laws established by the Church. There is a specific process for receiving permission to do so.
If Yes, does that mean that all people in the world in non-sacramental marriages are living in mortal sin?
You are confusing two different things.
A sacramental marriage is simply a valid marriage between two baptized people. A natural (non-sacramental) marriage is a valid marriage between two unbaptized or one baptized/one unbaptized party.
If a marriage is valid it is either sacramental or natural. Nothing sinful about a valid marriage.
An invalid attempt at marriage means there is no marriage at all, for example a Catholic marrying outside Catholic form without dispensation.
I have heard that legal married couples need to live “as brother and sister” while they’re in the process of receiving the marriage sacrament.
Does that have any exceptions? I have also heard that once they have children together they don’t have to live “as brother and sister”
Thanks!
All of these things are best handled through a discussion between the couple and the pastor.
Encourage your brother to make an appointment with his pastor.