People judge because we are tribal creatures. We all want to be at the top of the heap. We all want the most status. Status is judgment. If I can tell you you are weird and I can convince others of the same, then I have taken a step up.
Of course, that step might not last long. If people eventually realize they could be in the same position as you are, and that your sister is a conniving liar, the tables could turn, and she could be on the bottom of the heap.
Everyone has their own strategy for attaining whatever status they can. Some people are compassionate and others are vicious. There are many other strategies. One strategy might work at one time and not work at another time.
In your case, your sister wants you to recognize she has power over you and that she is higher in the pecking order—in your family, if not in general. By not buying the furry stuff, you essentially agreed that she has more status. You let her control your behavior.
On the other hand, by bringing the issue to fluther, you are seeking to create a groundswell of virtual support for yourself. This, in the long run, may give you what you need to stand up to your sister, and perhaps put her in a place below you.
I have to tell you that in the long run, these sibling rivalries usually turn out to mean nothing. Blood is thicker than water, and as you get older, in most cases, you and your sister will be on the same side against the world. Although, there will still be little battles between you to establish your hierarchy. It takes a lot of compassion and friendship to agree to be truly equal.