My first thought was [name]Vivian[/name], as suggested by pp, which I think ties in nicely with your sons’ names but still sounds distinctive. I would personally avoid another 2 syllable name with the stress on the first syllable just so try and vary the sound a bit. It’s not essential, though.
And if you decide not to stick with the -an ending I would suggest-
[name]Isla[/name]
[name]Ciara[/name]
[name]Harper[/name]
[name]Edith[/name]
[name]Maeve[/name]
[name]Adelaide[/name]
[name]Mara[/name]
[name]Lila[/name]
[name]Georgia[/name]
[name]Hazel[/name]
[name]Ivy[/name]
[name]Pearl[/name]
[name]Leona[/name]
[name]Violet[/name]
[name]Clara[/name]
[name]Esme[/name]
[name]Iris[/name]
[name]Delaney[/name]
[name]Olivia[/name]
[name]Grace[/name]
[name]Aria[/name]
[name]Nina[/name]
[name]Bethan[/name]
[name]Rhiannon[/name] (not an but close!)
[name]Marian[/name]
[name]Regan[/name] or [name]Reagan[/name]
[name]Helen[/name] (again not an but close!)
[name]Vivian[/name] is double-perfect. It’s a sweet, stylish name; it ends with -an; and it means “lively, life.” Since your boys spell out DNA, the very language of life, what a fitting capstone!