Just to parrot everyone else, my oldest was always ahead in developmental milestones, and extremely verbal at a very young age. My youngest appeared like a slug, and I worried so much about how she would feel compared to her brother. To the point that I argued with her incoming kindergarten teacher (who also had her brother) that she is NOT the same, and she needs not have too high of expectations for my daughter. She ended up surpassing her brother, making me look/feel like an idiot. She is just introverted, and more of an observer than a do-er.
My husband's brother was/is gifted, and he struggled in school. His parents focused on each of their individual strengths, for my husband it was sports and construction whereas his brother was academics. They also had different expectations for their abilities, and focused more on effort.