"I said in mine heart concerning the estate of the sons of men, that God might manifest them, and that they might see that they themselves are beasts.
For that which befalleth the sons of men befalleth beasts; even one thing befalleth them: as the one dieth, so dieth the other; yea, they have all one breath; so that a man hath no preeminence above a beast: for all is vanity.
All go unto one place; all are of the dust, and all turn to dust again."
abadonna, the above is taken from the Book Of Ecclesiastes, Chapter 3, beginning at verse 11.
Many Christians believe that animals are different than us. That they have no spirits... I'm a Christian Wiccan, and have different thoughts on the matter.
Tell your sister she should aproach the idea the same way she would as if it were a human deceased to be mourned. Thatb may be a good place to start.
Hope it helps!