Brief Overview of Wincest
At the begining of the show, it is clear that there is a lot of tension between Sam and Dean.
Dean feels betrayed by Sam because he left the family to persue college. Also that Sam does not realise how hurtful the loss of their mother was to John and Dean. Which is also why it is important that they find whatever it is that kiiled her and destroy it once and for all.
Sam on the other hand, feels that it is pointless to go after whatever is that killed their mother and that in doing so, they are separating themselves from humanity. He wants a normal life as is evident with him about to enter law school at the start of the show. However, with the death of his girlfriend, it seems more important now than ever to persue the demon.
Over the two seasons, their realationship has grown. They have learnt to trust each other again. But Dean still seems a little less open about his feeels and usually falls back into protective mode (just as he did when John went hunting when they were kids). They have become closer just as siblings should and have decided to "join forces" and destroy the demon that took the people they loved the most.(Their mother, father and Jessica).