Hello Friend, Good that you are seeking a help here. Marriage is a long term commitment with its own joy, responsibility,commitment, and chellenges. One need to mental prepared to accept it, else their adoption will cause trouble later. May be your sister is carrying some negative perception about marriage based on some incidents or  love failure or someone's experiences within her close circle. Better, that she gets a help of marriage or psychology counseling to remove this fear. Arrange for a counseling locally or if she hesitate to meet f2f the reach out us for online counseling.

There could be some reason for her refusal and fear.... Fix an appointment with a psychological counselor in your city for her... Probably, she might get a clarity and a better understanding, by sharing her emotions to a professional...