I really like the name
Tanith. It's one of the few T names I like. It's exotic-seeming, but doesn't sound self-consciously fanciful, to me. I hope your sister uses it! It seems to me like
Tanith would work with a very unusual middle name as well as it would with the usual ... just nothing that has noticeable hissing or buzzing sounds (F, SH, S, K, TH, X, Z) in it.
Tanith Medora...? (Medora is an invention of poet Lord
Byron, not in the database)