Hello Friend, Good that you are seeking a help here. Every one has a belief system which is deep rooted in based on experience. So it's better to understand the reason behind your sister's belief in order suggest her an alternative. She will carry such guilty if something goes against her beliefs if she is not convinced correctly.
She may need a support of marriage counseling to understand her expectations and reality.
Next Steps
Feel free to reach out to me online through Practo for further detailed evaluation and recommendation.