To see in the dark, you need three eyes. When the kind sister heals you after the evil one gives you the same deal she gave Odin, she gives you an extra as well. The evil sister is located in the upper left third of the central heart-- try using Love or Wind where the three shades go up and down together.
Viewing post in Funeral Song for the Elemental Lords comments
Ok, sorry. From start: go right to the staircase, then jump left over to where the attacks are coming from. You need to jump over the attacks with good timing to make it to the screen to the left. There you will see an enemy jumping up and down and shooting the attacks. You can jump under them with good timing, or deal with them any other way, but you need to go left. From there you will need to use fire, love, or wind to cross the chasm by jumping across with recoil, turning the enemy into a platform, or jumping up from the bottom respectively. You are trying to go left without going up. When you do this you will see a row of three enemies moving up and down in a short hallway. Turn them into kittens with love and ride the middle one up to find the 'evil' sister (you can also make the jump with wind, but it's a little tricky since you have to dodge the enemies as well then). She will cut out one of your eyes and eat it, but in the process give you the power to see what's hidden-- false walls will change color so you can tell where they are.
From there, go to the other sister in the east. To find her, go back to start, then go right, up the stairs, then go down and left through the fake wall below that you can see through now, dodging the attacks the enemy on the platform shoots. Follow that path into the Tearstained Catacombs (purple walls, True Lord of Wind's area), then walk right through the checkpoint, through a fake wall on the top right, to a room with a ladder. Climb the ladder and you will see a fairy (if you can see fairies) and a woman. Talk to the woman exactly once and she will give you three eyes, which lets you see in the dark. If you talk to her again, she'll restore your eyes to normal and lend you glasses (four-eyes is the pun), which have no effect, and you can give her the glasses back by talking again to return to normal, at which point you'd need to get an eye eaten again to be able to return to seeing in the dark with three eyes.