I would feel sorrow for your brother and his girlfriend for falling into sin, and sorrow for them and your parents not knowing the truth. I would redouble my prayer and fasting for their conversion, and pray daily to the Holy Spirit to grant wisdom to know when and what to do and to say. On top of that, I would consecrate them to Our Lady so that she will be their mother and help them convert.
I cannot approve what God does not approve. So I will not condone what they do. However, I must not treat them with hostility. The key word for helping people to see the truth is to invite them to see how much true joy there is in living the truth. So while I have sorrow for sinners, after I have entrusted my family to God, I must trust Him to bring about their conversion. I must then live my calling with love and joy and peace.
I do not know how well your relationship is with your brother. If it is good enough, he needs to know that you love him. Get to know what he is thinking and invite him from time to time to consider things, things that are positive about the faith. Do not accuse or judge. As the Lord said, they don’t know what they are doing.
I also don’t know how old you are and if you are financially independent to move out. If things get really really bad, that may be a necessary option.
Lastly, get to know his girlfriend. Everyone has a story. Find out the beauty within her, and draw out her goodness. When the right opportunity comes, introduce her to Jesus whom you love. Do the same also with your brother and parents. When we testify to the wonders of God in our life with love and authenticity, miracles happen.