Based purely on sound, I'd go with Alexander or Luke. Amelia and Liam sort of blend together for me. Probably Alexander because it is more symmetrical with Nicholas (long formal w/ common nns).
For a girl, I'd go with Katherine. Alexandra's fine, too, though it gives you two A names (not a big deal). Morgan is an odd fit because it is androgynous where Nicholas and Amelia are solidly masculine and feminine.
No bad names here, though, so you can't really go wrong.
Re: Sibling for Nicholas and Amelia
Baby Name Popularity by State
Katherine and Luke.
Suggestions would be Natalie, Camille, Caroline, Veronica and Vivian for a girl and Andrew, Calvin, Nathan and Samuel for a boy.
GL!
BFP 1- EDD 2/09/11 Missed MC DX @11 weeks D&C- 7/25/10 BFP 2- EDD 12/22/11 Natural MC @ 5w 2d BFP 3- EDD 1/25/12 DD Josephine born 1/16/12
Boys:
Liam Alexander - I love the name, but it has too many of the same sounds as Amelia
Alexander Marcus - Not bad, but don't like siblings starting with the same letter
Luke Matthew - Dislke Luke
Girls:
Katherine Claire - Love
Alexandra Quinn - Nice name, but I don't like both girls starting with "A"
Morgan Hadley- I've been watching too much Merlin, and Morgana is the evil chick, so that's all I can think of
Other suggestions for boys:
Sebastian, Harrison, Simon, Grant, Evan, Cameron
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Carter Robert 7.18.08 | Brynn Sophia 5.24.10 | Reid Joseph 9.10.12 | Emerson Mae 1.27.14