For most characters, motivations are driven by family of origin issues. This includes far more than one's relationship with parents. Sibling relationships and one's place in the family pecking order can be strong influences on characters.
Dr. Kevin Lehman's The Birth Order Book looks at the particular pressures of being first born, middle, last born or only child. He goes on to discuss how the family dynamic tends to shape relational styles and personality development for each birth order position. He concludes that these early relationships shape not only the family dynamic but also how each individual relates to people outside the family. He observes that first borns and only children tend to be achievement oriented, natural leaders who desire affirmation from authority figures. Middle borns, he says, are laid back, excel at mediating and developing consensus and tend to be more open with friends than with family. He observes that last borns are creative, rebellious and often rely on humor and charm to get along in the world.
I'm not in any position to critique the science here, though more recent studies, like the ones reported in this Time article, do seem to back up his observations. I've simply found the book helpful in the character development process. When I think through the backstory of any character, birth order has a place and can subtly bring verisimilitude to the story. In Bring to Light, when I needed a likeable guy friend character who would respond with gentleness and humor to my protagonist's plight, I made him a last born with older sisters. My protagonist's workaholic mom is, of course, a first born. Her laid-back dad, the youngest of two.