I would pronounce
Arianne and
Ariane differently. I'd pronounce
Arianne like 'air-ee-ANN' and
Ariane like 'ahr-ee-AHN'. I definitely prefer
Ariane. It just looks neater and more legit for some reason.
I do think that Arianne/Ariane would definitely be too close to use if the older sister is called
Joanna. :(