I vote for Evie May, because it's sweet and southern sounding. But right there in second place is Adah May, which I also think would be hard to say without a twang : ) I like Mae more than May, but only because of my irrational dislike for Y's
Eve May Katherine is my favorite by far from your other choices, but mostly because Reveley & Everhart do not have the southern charm that is so necessary for the NN Evie May. I think Evangeline or Genevieve would be beautiful with May Katherine, though : )
I LOVE EmmyLou; I always have. Its a name close to my family, especially my mother.
Memphis Louise is so cute, plus I think Memphis & Shiloh go pretty good together. I think adding Katherine at the end throws off the flow of the name, but I understand wanting to honor family (as I'm sure that's what you're going for). If you're not too fond of Memphis, why not just do Emma Louise Katherine, or EmmyLou Katherine? Emma & Shiloh is darling, even EmmyLou & Shiloh!
I think Reveley doesn't make sense. Reverie does, though, plus it actually has a lovely meaning and gets you to Evie even better. Reverie & Shiloh are adorable. Reveley just looks awkward and it doesn't mean anything. If you didn't like Reverie, Revelry is close to Reveley.
Euphemia is really cool too, but I don't know if it goes with Shiloh very well. Shiloh is a very down to earth name, and Euphemia is over the top in comparison. Shiloh & Effie is cute though.
Anne is too plain next to Shiloh, and next to most names in general.
I prefer Ruthie rather than Ruth. Ruthie & Shiloh just falls flat, though.
Nell & Shiloh would be cute. Maybe not Helen & Shiloh, but Penelope & Shiloh. Darling and quirky yet southern, and the nickname can be Nell or Nellie. Penelope May Katherine?
Tallulah just popped into my head, too.
To be honest, Emma Louise Katherine or EmmyLou Katherine is my favorite. Memphis Louise is very cute though, too.
Lou, May, Nell, Emmy, those are all very Southern nicknames. Love them all.
I love Emmy Lou and think Memphis and Shiloh are a great sibset. Effie May is my guilty pleasure on the list but I am unsure about Euphemia. Your could use a fn that starts with F (Frances, Faith, Freedom).
I liked the suggestion of Tallulah as well. Tally is the perfect Southern girl nn, and you could have Tally Mae.