Holden and Allie are best friends, besides being brothers. Their connection is pretty close and intense during the time Allie is alive, and Holden's connection to his brother maintains its intensity after Allie's death.
Since Holden and Allie were only two years apart, they were able to share a lot of their growing up processes, and from Holden's own words, we can tell that they were each other's main support systems.
When Allie dies in the family's summer home, Holden breaks his hand breaking the windows of the garage. He then he is sent to what seems to be a psychiatric hospital, making him miss the funeral of his beloved brother. Perhaps this is where Holden's desire to hold on to the memory of Allie becomes stronger, and his convictions become more deeply anchored.
Allie represents in Holden all that is innocent, sacred, pure, unique, and charming about life. Not having him is more than just a grief and loss process for Holden, but also it is one of the toughest tests that he faces as he becomes a young man. In all, the brotherly love between them was intense enough to endure even after death.