The only sin God can't forgive is the sin of rejecting His offer of forgiveness through Jesus Christ. God's promise is for you: "If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness" (1 John 1:9).
Don't, however, take God's forgiveness lightly -- because it was only made possible at the cost of Jesus' life. God doesn't forgive us because we're good, or because sin isn't serious. Sin is an offense against God -- an act of rebellion against the One who created us. In other words, when you wronged your cousin, you weren't only offending him -- you were offending God.
But in spite of all the sins we've committed, God still loves us, and on the cross Jesus Christ took upon Himself the judgment for sin that you and I deserve. Why not accept His gift of forgiveness by confessing your sins and committing your life to Christ today? Let the Psalmist's prayer become yours: "Wash me, and I will be whiter than snow" (Psalm 51:7).
Your dispute with your cousin was unfortunate -- and I hope others will learn from it. But it also points to another issue: the danger of greed. On one occasion, a man urged Jesus to demand his brother split an inheritance with him. In response, Jesus warned, "Watch out! Be on your guard against all kinds of greed; a man's life does not consist in the abundance of his possessions" (Luke 12:15).