This is probably a real career break for your daughter. Whether for business or personal reasons, your sister did not view your daughter as her first choice. If she had gone ahead and hired her, the stage would have been set for a failure.
If you feel that this situation is going to undermine your relationship with your sister, you should talk to her about it, but be sure to keep an open mind. Although it is possible that your sister is harboring some strange vendetta, it is also possible that she is a solid businessperson who found a very strong candidate and made a business decision. In fact, when it comes to family business matters, employers are consistently advised to focus on business, not family.
Your daughter may also want to talk to your sister about what happened here. If your sister's decision was truly based on business-related factors, perhaps she can give your daughter some additional pointers and career guidance.
In the long run, your daughter is going to feel better about herself and her work if she gets a job on her own. And, if your sister in fact rejected your daughter for personal reasons, she has actually hurt herself.
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