Kyokukou Senki Mistletear (Visual Novel)

A bizarre threat suddenly kidnaps two sisters: Akira and Kaede.
Akira wakes up transformed by the experiments of an organization called "Eitraizen".

Akira's one and only goal is to rescue Kaede,
an epic mission aided by her strange new superhuman powers.
She can now transform into "Blazing Angel Mistletear",
a holy war princess with godlike power... to make her enemies pay!!

[From DLsite]