Decided to give your post the benefit of doubt though considering your a newbie, don't know whether you landed at the wrong port.
@Mzazii is right on the money. I'd venture your facing an unconcious bias. At a certain level, women are the same. At another level there are many more successful, creative, attractive, sexy women than your wife's sister for that matter.
Only difference between humans and animals is intellect - so use it. Shape your environment (get her out of it) and you don't need to get along (no need to rationalise otherwise). And believe it or not, YOU most probably married the better sister.
Further reading
http://www.slate.com/id/2227808/