When I think of kid I think of like 4-9 when they have no sense of responsibility, at 14 your sister should be old enough to care for a rabbit, but if you are looking for a good argument ask her what she is going to do with it when she gets bored with it. Ask your mother what she will do with it when your sister eventually moves out. The adoption agency will hopefully help drive home just how much responsibility having a rabbit will be and your mother will ultimately be responsible for it.
There is only so much you can do about it if your mother is supporting your sister. Hopefully your sister has learned from her past experiences and will take better care of the rabbit.
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