As much as i love Ed and Sarah's sibling dynamic, i must said that i found really touching how Double D knows Ed better than his whole family.
Just imagine them being older, Sarah it's in a bit better terms with her brother now and she tries to understand him better...
'Ugh, Ed keeps yelling at night! The idiot keeps saying there's something under his bed, he is not five anymore'
'It's not because of that. Ed probably has something stuck on his right ear. He sleeps on his right side so he thinks there's a monster trying to suck his brain.'
'... Oh.'