It can be trying to have a sibling, especially during the adolescent period, when in fact one is discovering who one is. You live in an environment whereby boys can do as they please, as long as they do not bring the problem home. If the family has not had any strong involvement in his ethical upbringing, then this will leave your brother, and other boys like him prey to external influences, and the kind of influences that can claim control over his nafs (lower self). The school years can be a very trying period, especially if the values at school contrasts drastically with the values at home, however, if the home does not take responsibility for this while a child is growing up, then the school becomes the major influence in his life. Then added to this there are peer pressures, which can encourage one to not behave according to ones best interest.

Your brother’s nafs has become prey to the aspect of himself which is known as an-nafs al-ammârah. This is the part of the self which is demands much of the lower nafs. In fact, the lower nafs commands the person, rather than the other way around making the person a slave to their lower desires. The an-nafs al-ammârah thirsts for pleasure via lust (shahwah) and passion (hawa’), which explains why your brother is unable to commit himself to being better – he is not in control. If sustained the an-nafs al-ammârah compels him against the guidance of Islam.

The above does not mean that you give up. It is even more important that you become his mirror image, and keep warning and guiding him with compassion, and not as if he is a criminal. It is important that you do so that if the moment arises whereby your brother finds himself at the crossroads of doing good doing  and doing harm, that within his subconscious there is something to help him pull back from doing harm. It is unfortunate that your family was only persistent about separating from the Christian girl, and not persistent from separating from the Muslim girl. Unless he intends marriage, and that has been processed by both families in terms of engagement and/or preparation for marriage, your brother should not be allowed to be with any girl.

Currently his lower desires are completely out of control, so to expect anything of him, might be asking too much.  For your brother to regain control, he would need an external impetus that seems quite unrelated. It would be helpful if you could encourage your brother to take on a physical discipline, has that would help to mature his psyche. By this I mean some form of martial arts (not karate, but a true martial art) whereby he could learn to release, redirect, and transform his negative energy in a practical way,and to a higher level.

Be there for him sister, be his sister, and insha-Allah, it will not be in vain. Also, all the family should make regular du’aa for him.

For further guidance, please try the following link(s):
Fatherly Advice: Raising the Rebellious
Living With the Enemy
The Challenge of Educating "Troubled" Muslim Youth in America: The Taqwa Gayong Academy