If she is not losing weight I would keep trying to get her to eat on her own. She may be too spoiled to do so or as you suggest it may be a defensive response and as such the handfeeding is actually a negative. How long will your sister be gone?
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2.0 Leos (Spot - Normal but oh, so sexy; Jessie - hypotang)
3.2.3 Fattails (Henri, Suwannee, Valencio, Dixie, Isis, Pheonix, Hissy, and Snickerdoodle)
0.0.1 redfoot tortoise (Sherman)
0.0.1 sulcatta tortoise (Tinacious aka Tiny)
1.0 Dog (Zubin)
0.1 cat (Mimi)