I see Susan as more of a Taurus or a Leo than a Cancer, because she's stubborn and hot-tempered and convinced she's always right. But! I guess there are some traits that fit her (well, that's true of every star sign, which is why horoscopes work for so many people).
The general traits for Cancer immediately caught my eye because of "the ability to express emotion, intensity, tenaciousness" (supposedly positive) and "bossiness, emotional manipulation" (negative). The Hufflepuff-centric Cancer description doesn't fit at all - except maybe the "need to be needed" bit?
Anyway, Susan thinks that horoscopes and star signs are a load of old rubbish (not surprisingly, I suppose - but also because that's what Amelia thinks, and Susan shares a lot of Amelia's opinions).