日本語 Japanese Ask Questions Thread 3

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Japanese Ask Questions Thread2
http://dis.4chan.org/read/lang/1206158123/1-40

I finished katakana a couple of days ago, and have started learning the grammar. I'm still at particles though, as something really confuses: the が particle.

ボブは魚が好きだ。
Bob likes fish. This much I can understand, as が is referring to something that was being talked about, ボブ.
Replacing が with は would turn this into "Bob" "Fish likes" I assume. Then:

俺の妹がこんなに可愛いわけがない
Why is が used here? Isn't the topic "俺の妹"? Why not は?

I'm not getting this at all, honestly. How do you usually use it? Practice, or by rule?