When (and How) to tell kids their brother has a different father?

With as little editorial as possible, here is our situation: When I met my wife she had a son. She never married his father, but Tim sees his dad every weekend for 12-24 hrs. My relationship with Tim is great; step-dad with the emphasis on Dad. Now there are two more kids in our family, ages 2-1/2 and 5 mos. They don't know yet where Tim goes every weekend or why, but the toddler has started asking. She is very bright, but we're not sure how to approach her with the idea that her brother has a different dad. We have friends in a similar situation who told their toddler too young and struggled for a while. The child guarded the relationship with the parent that was his alone and didn't want the sibling to be close to that parent. Our daughter idolizes Tim, and we don't want to do anything to jeopardize that. Still, it feels like the "where is Tim" questions (and occasional tears) are saying the time is at hand to tell her. Tell her everything? A little? or Wait? - and then, what to say?