It is great that you found the passenger list. What were the names listed? Were there the parents and three children? Also, if you have not found a death or burial record for the mother, how do you come up with 1851-1899 as well as the surname of Denuit? I found a Josephine Denuit on the Mormon site. She was born 6-12-1849 in Glabais, Brabant, Belgium. Her parents were Dominique Joseph Denuit [3-25-1813, Glabais] and Marie Therese Decamp [1-14-1811, Glabais]. If Alexandre's mother was Josephine Denuit, then this person is likely her. Her six siblings are listed also.
You mentioned not finding a death record for Josephine. Deaths in LA were not officially recorded until about 1911 [I think]. So, the only record would be a burial record. Did you consider that the family lived elsewhere before going to Rayne. Did they come to the port of New Orleans? If so, she may have died in the city before the family made its way out to other parts. I didn't find her in the NO records, but it may be that she died there but it was not reported.
I found Charlotte's tombstone at "Find A Grave" as well as that of Victor. Now that you know that Emile died in 1925, will you be able to check the burial records at the church? That is an interesting story about him being buried at night!
This is interesting research!
Do the Lepine's speak French?