If your girlfriend’s brother is Catholic, and getting married in ‘another Christian’ church (unless he has been given permission by his bishop to do so), then he is not marrying validly in the Catholic Church. And while nobody is going to stand guard and remove a Catholic from the wedding, it would be wrong to attend. Morally wrong. Can you find out if (if the groom to be is Catholic) he has indeed received permission?
But if your girlfriend’s brother is not Catholic, and has never been married before, there is no problem. Giving the readings though, could be problematic. Usually, people who are more fully ‘participating’ in a liturgy are perceived, rightly or wrongly, as more supportive of that liturgy. But depending on the liturgy and depending on the reading and circumstances it might not be a problem. Of course, as above, if the groom is Catholic, does not have permission to marry outside the church, then since one should not attend an invalid wedding one would not even BE there to do the readings.
Another point that may cause problems is that a Catholic may not ‘take’ a communion in a non-Catholic church. neither may a non-Catholic receive communion in a Catholic Church except under circumstances extremely rare and necessary to consult a priest for; further a Catholic not in a state of grace may not (morally) receive communion.
But as I’m sure you know, the fact that there are things which we ‘should not’ do or are morally forbidden to do does not stop many people from actually doing them, encouraging others to do them, etc. I guarantee that you’ll find scores of people, even (sadly) priests who will bleat that it is more important not to ‘disrespect’ the family and that hey, we’re all ‘one’ where it counts --and who call those who disagree with them bigots, unChristian, pharisees, etc.
If people think of Catholicism as ‘rules made by men which need to be bent, or need to be relative’. . .they’ll be more inclined to ‘break’ or to ‘interpret’ them.
If people think of Catholicism as ‘the guidelines given us by our Loving Savior, to be followed because of love of God and our deep and abiding trust in Him, even if difficult, even if OTHER PEOPLE make fun of us, or call us names, or attempt to make us look intolerant’–they might be more inclined to stop to think that if GOD says ‘X’, when everybody else says ‘Y’, maybe we should pay a little more attention and have a little more faith in GOD instead of in fallible, changeable, sinful men. . .
