Hello dear I appreciate your concern to your sister and courage to approach the experts for help her.
Taking decisions about marriage isn't easy, there could be lot of confusion and confidence issues she could be going through. If she is not comfortable in sharing with family members, she can surely take the help from counsellors or marriage counselor. In which she can take premarital counseling, she can share her concerns based on the same she will be guided for the best.
Her physical symptoms are sounding because of anxiousness she is experiencing. It's ok no need to worry that can be taken care.
Next Steps
please look for the marriage counselor or counsellors for better guidance and well being. take care.