Long hair can change someone's appearance rather drastically, especially when it covers the ears and a bit of the face on the sides like Lenne's does. I think they look as similar as Tidus and Shuyin do, it's just that the long hair makes her look that much more different.

Yuna and Lenne are connected, just like Tidus and Shuyin are. And the reason why Tidus and Shuyin resemble one another is probably the same as why Yuna and Lenne resemble one another.