The context of Cain's inquiry was that of a guilty man filled with anger, jealousy, conviction, and regret. But what about the context of this question in the church today? Am I my brother's keeper? Is it my responsibility to guard, protect, and watch over my fellow believers and make sure they don't fall into sin? We don't need to go far in the scriptures to see an example of this. In Genesis 3, the first humans were faced with a temptation. Eve was beguiled by the serpent and ate of the tree that God expressly prohibited them to eat from. The blame, however, can not fall solely on Eve. Genesis 3:6 reads: "And when the woman saw that the tree [was] good for food, and that it [was] pleasant to the eyes, and a tree to be desired to make [one] wise, she took of the fruit thereof, and did eat, and gave also unto her husband with her; and he did eat." Eve was not alone while facing this temptation. She had Adam there, who also knew right from wrong and what God had commanded. Adam could have (and should have) stopped Eve and got her back on the right track, but instead he stood by silently and even fell to the temptation himself. Letters to the church in the New Testament address this topic as well.
Galatians 6:1-2 NKJV -  Brethren, if a man is overtaken in any trespass, you who [are] spiritual restore such a one in a spirit of gentleness, considering yourself lest you also be tempted.  Bear one another's burdens, and so fulfill the law of Christ.
James 5:19-20 NKJV -  Brethren, if anyone among you wanders from the truth, and someone turns him back,  let him know that he who turns a sinner from the error of his way will save a soul from death and cover a multitude of sins.
These are just two more examples in scripture that the church is expected to support, help, strengthen, guard, and restore one another. We should be continually covering our brothers and sisters in prayer. If we see them heading toward danger we should speak up and reach out a helping hand. What a shame it would be if we were to stand by silently while a precious believer plunged into the depths of sin and death. If they do fall into sin, we please the Lord by loving and restoring them back into the fold.
This doesn't mean that I can live carelessly and expect the church to be solely responsible for my salvation. Besides being instructed to guard and restore one other, we are also told in Philippians 2:12 to dutifully and reverently work out our own salvation. These two ideas don't contradict one another. I am responsible for my walk with God, but I'm also expected to be there for my brothers and sisters if they fall into temptation or begin to stray off the path. Ultimately, each of us is responsible for our own salvation. However, should a brother or sister walk away from the faith we must not answer God "am I my brother's keeper?". We should rather answer "I did all I could do to help them".
One of the greatest blessings and assets to a believer is the church. Plug yourself in as much as possible. Love the church, prioritize church attendance and fellowship with other believers. This road is much easier, more successful, and more enjoyable when we walk it together.