Autumn, if I was you I would just try to approach the subject gently and assure this person you are not trying to interfere but you are only concerned about their happiness.
Lord, I pray You would give this family member clear visions of how difficult this marriage will be when one person is becoming more like Jesus Christ and the other is taking a different path in life. I pray the fiance would become saved. I pray You would give Autumn the words to say to confront this person in a loving way. Thank You, Lord, that You are capable and willing to do all of these things and that Autumn cares so much for this family member that she would call on You. In Christ's Name, Amen.