Pete and Donald are both sympathetic characters. Pete is practical. In his mind, this is a strength; in Donald's mind, this can be a flaw when it is Pete's only motivation. Donald is a free spirit. In his mind, this is a strength; in Pete's mind this leads to irresponsibility.
Pete has spent his life doing things by the book. He doesn't waste time on religion or anything he thinks is impractical. It seems safe to assume that Pete stays too busy to be philosophically introspective about his life or about the lives of people around him. He thinks of his wife, kids, and Donald only inasmuch as they depend upon him. His entire reason for being is to be responsible and provide for (reluctantly for Donald) those in his family. One could be sympathetic towards him because, in his stubbornness and extreme practicality, he doesn't consider deeper meaning in life. With respect to his brother, Pete never really considers that Donald, although wandering and financially irresponsible, might have a perspective on life that Pete would benefit from. Pete's flaw is his lack of depth and his automatic adversarial approach to Donald. This stretches back to their childhood when Pete would punch his brother's stitches. Pete is a sympathetic character in that the financial burden has always fallen upon him. But Pete continues to indulge in this role, perhaps out of a feeling of superiority and perhaps out of habit. In this respect, it is hard to sympathize with him.
It is hard to sympathize with Donald's complete lack of responsibility and how he always counts on Pete to bail him out of a bad situation. However, Donald's irresponsibility can be attributable to him being a free spirit. And with this, Donald is quite generous, even with Pete's money. In the end, Donald is a more sympathetic character because his generosity equals or supersedes his lack of accountability to his brother. Not to mention, it seems that Pete was antagonistic toward Donald since they were children. Donald is also more sympathetic because, unlike Pete, Donald never found a stable place in society where he truly fit in. Pete has criticized Donald's lack of stability, essentially criticizing his way of life.
Then again, Pete might be the more sympathetic character. He does show very subtle indications that he could use a lighter perspective. In particular, Pete almost tells Donald about his dream:
"It was strange. You were taking care of me. Just the two of us. I don't know where everyone else was supposed to be."
Pete left it at that. He didn't tell Donald that in this dream he was blind.
This could suggest that, underneath it all, Pete did want a better relationship with Donald, one in which he could depend on Donald in some way. It could also indicate that Pete was suffering from a spiritual or ethical blindness and secretly felt that Donald might shed some light on such a subject. Given these possibilities, even though Donald seems more socially lost, this would indicate that Pete is lost as an individual. But Pete is too stubborn to treat Donald as an equal and bridge that gap between his practicality and Donald's spirituality. In this case, one could be sympathetic to Pete's stubborn mental block. One the other hand, one could argue that Pete should stop being so judgmental.