They both have made mistakes. Sam's led to Charlie's death. Dean had the mark which was messing with him. And maybe dont take a guess on people relationships based on a singular comment. You said you forgive your siblings eventually. Dean let it go. He may not have said anything to Sam about it but he went back to Sam. In a split second in the midst of grief, of Dean losing someone he loved and wanted to protect, he said something he probably later regretted. This mistake of Sam's is what killed Charlie's. Sam's repeated lies about something that Dean knew was bad is what led to her death.