A question about the に particle

I have a question about the に particle.

I was never told that it was possible to do something like this:

私は妹に新しいおもちゃを買いました。

I thought you HAD to say 私は妹のために新しいおもちゃを買いました。

よい子【こ】になるには早寝【はやね】早起【はやおき】するコトです。

How come に is used with a verb here like なるに?

Is that possible too?

How often am I allowed to do that?