This dream is an invitation for you to pray...
You see that people younger than yourself are out of control, with no one paying attention, no one to protect and guide them. Your grandparents' house and your uncle indicate this is linked either to your actual, bloodline family or your spritual/church family. The "huge wave of sorrow and worriedness" reveals your compassionate heart.
You have a gift from God to pray for the needs of others as He shows them to you. What an honor! In this dream, God is inviting you to pray in agreement with Him that the older and more mature in families - both natural and spiritual - need to wake up and respond to the needs of the younger generation.
If this strikes a chord in your heart, ask God how He wants you to pray and He'll give you more specifics. But don't let this dream continue to cause you fear and anxiety! Those emotions are not from God, so don't turn your attention to them. This is not a literal dream that your cousin is specifically going to die, OK? Trust God and Him alone, and you'll find peace instead of fear!
About the satanic implications being posed...
The number "six" in many dreams represents mankind in general. the mention three times in your dream indicates that God (triune Godhead = 3) has given you the dream. This is an intercession dream. If you trust in God to lead you, He will give you more dreams like this so that you can agree with Him about things that matter to Him. He won't impose His will upon the earth, but if we pray in agreement with Him he can move powerfully on behalf of people even when they don't know they need His help!
Again, this dream is truly an honor toward you. You have found favor with God!
Dogmatic religion vs. relationship with God through Jesus
Dweezil's comments reflect the widespread misunderstanding about true Christianity. Following dogmatic rules in order to please an unapproachable god is not what being a Christian ("little Christ" or "little annointed one") is about. In fact, Jesus spoke sternly to the religious leaders of His day because they kept loading endless requirements upon the people, but did little or nothing to help them. The concept of following rules and rituals in order to please - or appease - an angry, distant "deity" is actually a pagan (or ungodly) practice.
Jesus was never about laying heavy burdens on people, or holding them to impossible standards. He said, "Come to me, all you who are weary and heavily burdened, and I will give you rest." He taught that patience and forgiveness toward others pleases the One true God more than any actions on our part. In fact, we can't earn His favor - He's already given it to us, free and clear. All we have to do is accept what He's already done for us. When we come to realize what that is, I can tell you firsthand that you cannot help but love Him more and more.
Jesus is about healing our hearts and sharing His love with us, and extending that love through us to others. He is not a harsh taskmaster! Again, I say this from personal experience.
If you only knew what He's done for me...