Ask Mr. Smarty Plants

Here are a couple of previous answers to similar questions:

We found a previous Mr. Smarty Plants answer that comes about as close as we can for an answer to your question. The plant mentioned in that answer is non-native to North America, so we don't know where you could get it. You might try going to our National Suppliers Directory, typing in your town and state in the Enter Search Location box, and you will get a list of native plant nurseries, seed suppliers and landscape consultants in your general area. Most of them are not exclusively native plants, and they all have contact information so you might be able to get in touch with some of them and see if they either have it or know where you can get it.

Pictures of Epiphyllum oxypetalum from Google 

Second Answer:

Mr. Smarty Plants didn't find a plant with the exact name your grandmother used but here is one possibility—Epiphyllum oxypetalum (Night blooming cereus, Dutchman's pipe cactus, Queen of the Night). A couple of people commenting on this plant referred to it as "Babe in the Manger."  I found other references to that name associated with Epiphyllum oxypetalum on several plant forums as well.  You can search on the linked page given above for "Babe in the Manger" or scroll down to the entries by NEVADASKIDS or lindas43.  The first entry says her mother called it that because the bloom looks like "Baby Jesus in the Manger with the star at his feet."  Epiphylllum oxypetalum is a member of the Family Cactaceae (Cactus Family) and is a native of Mexico and Central America.  It does bloom at night and is fragrant.   Here are more photos.  Mr. Smarty Plants thinks you need to have a pretty good imagination to see the baby Jesus in the center of the flower.