Well, one thing would be the fact that she didn't use the Skywalker name.
Second, Anakin was never told he had twins. So for all he knows, he only had one child...and when he learned about Luke, he didn't even think that there might be...(hang on)
"There is...another...Sky...walker..."
Sorry, had to do that.
Also, the fact that she focused more on doing politics she didn't even think that she might have special powers...whereas Luke had all the time in the world between Harvest and such to have a slight feeling that he may have such powers.
...anyone else want to add to that?