It's pretty, and the pronunciation isn't too difficult. I'd say that most people have at least heard of the flower. The name is long, but she could always use a nickname.
Amaryllis Gwen has a great flow.
Amaryllis Blythe works as well.
Amaryllis Ludivine really caught my attention. It's long, but lovely. In general, I'd recommend a shorter middle name (1-2 syllables), but it works. So does
Amaryllis Coraline.
I'd avoid
Amaryllis Imogen and
Amaryllis Winifred because they contain too many of the same sounds, and the combos are long on top of that.
I'm not really a fan of themed sibling names, so I wouldn't recommend using
Hyacinth for a sibling unless one was a middle name and the other a first (example:
Amaryllis Gwen and
Imogen Hyacinth).