I read one of your answers for a similar question, and I was hoping you could help me with mine: I'm 17 and I have a younger brother, age 14. The past few days he has been using really bad words --words that would make anyone angry. He has been avoiding me, constantly fighting, and annoying me. When he is with his friends he talks bad about me with no second thought to what he is saying. He says incredibly mean stuff to me, and I constantly tell him to avoid using bad words.
I am an ardent Christian and I believe I should alway forgive my brother, but I want to stop his behavior, so please help me out.
Since each person is responsible for his own decisions (Ezekiel 18:20), there isn't any way that you can make your brother do what is right. You can't force him to change. Trying to do so will only get you frustrated and angry.
Since you are not his parent, you can't punish him for misbehavior either. If your own parents choose to ignore his behavior, then you can't make up for that lack. "A foolish son is a grief to his father, and bitterness to her who bore him" (Proverbs 17:25). Their tolerance of bad behavior is their choice, though it too is a bad choice.
The only thing you can do is encourage him to want to behave better. "Now we exhort you, brethren, warn those who are unruly, comfort the fainthearted, uphold the weak, be patient with all. See that no one renders evil for evil to anyone, but always pursue what is good both for yourselves and for all" (I Thessalonians 5:14-15). In other words, getting into arguments with him isn't going to convince him to improve. "Let no corrupt word proceed out of your mouth, but what is good for necessary edification, that it may impart grace to the hearers" (Ephesians 4:29).
What you need to do is think, from his point of view, why he would want to be better. "Now why would I want to make you a sandwich when you talk bad about the all the time?" It is small reminders like this that put what he is doing into perspective.
Much of what is going on is the stage he is going through. He is starting to mature, but the part of his brain that does judgment and restraint is still immature. He blurts things out without thought of the possible consequences. He'll eventually grow out of it.