I'm not entirely sure, but it might have originated from a certain religious community's need for a song to convey a particular religious story or sentiment.
The significance might lie in the family's religious practice. If they are Christian, 'I will sing the wondrous story' is a hymn that tells of God's love and salvation. The Zehr family could be using this hymn to pass on their faith to younger generations.
It is a song often heard in religious or spiritual contexts. It has a very positive and inspiring message, probably about sharing a great story through singing.
The phrase 'I will sing the wondrous story' is often associated with a hymn. Maybe the Zehr family has a strong affinity for this hymn. They could be involved in a church or community where this hymn is regularly sung. It might be that a member of the Zehr family has a beautiful voice and loves to sing this story - the wondrous story might refer to the story of salvation in a religious context.
It could be a story about the power of friendship. There could be a group of misfits who come together and through their combined strengths and the bond of friendship, they are able to achieve great things. It might also be a story of self - discovery, where the main character goes on a journey to find out who they really are and in the process, uncovers amazing secrets about themselves and the world around them.
A good start could be with a strong statement or a thought - provoking question. Such as 'In a world where magic was thought to be long gone, a strange event was about to change everything.' This creates a sense of anticipation. Or you could start with a description of the mood. If it's a spooky story, start by talking about the cold wind and the howling of the wolves in the distance.