Walter and Benetha: Nearly Everything
Walter and Benetha, despite the fact that they are brother & sister, do not get along under any circumstances.
There are few situations where Walter will not criticize Benetha (I.E. her 'African Heritage'), nor is there a situation where Benetha agrees with Walter (I.E. 'Liquor Store Plan')
This could've come from two likely situations: As a child, Benetha was treated better than Walter (because she is so much younger), and Walter was jealous of this, or simply because they are in such confined quarters.