I've often said that there's no such thing as writer's block; the problem is idea block. When I find myself frozen-whether I'm working on a brief passage in a novel or brainstorming about an entire book-it's usually because I'm trying to shoehorn an idea into the passage or story where it has no place.
If you don't understand how a woman could both love her sister dearly and want to wring her neck at the same time, then you were probably an only child.