The Sisters Strategy
I went into the last battle with only 7 hearts and 4 energy bolts, and manage to take down the sisters. It was really not much of a problem, you just need to practice to handle the 6 different phases flawlessly.
First phase you'll be fighting orbs while the sisters will send out bolts of alternating colors. I've found it easiest to take down these orbs going in a clockwise circle. Just make sure you don't loose any hearts on this easy step.
Second phase is where you can damage the sisters. Each sister can take 9 hits before she becomes invulnerable. Start with the red one, and end with blue - because you would like to end up on the right side of the screen for phase 3.
Third phase is a bit tougher. Get to the outer circle as quick as you can and travel up with it. You will have an easier time to fight off the outer orbs on top. When you killed off all outer orbs, move in and do the inner orbs. I usually do this on the right side, but I don't think it matters.
Fourth phase is the second time you may hurt the sisters. Begin with the blue sister and kill the red one last. This is to be positioned favorably for the fifth phase.
Fifth phase is hard. Get to the outer circle immediately and use it to travel up. You will have easiest time killing of the outer orbs while on top, just like the third phase. The hardest part is to kill off the inner orbs. You might have to spend one of your powers here.
Sixth phase is the last one. You'll get to kill the sisters. I had saved all my powers to this phase and killed them off with my light ray - staying on a comfortable range. Melee on this phase does not look impossible, you just need to be careful and switch color as needed.
Congratulations on finishing the game!