I like a straight forward arcade shooter! The idea of the two spaceships un-linking is cool, and having to join up with the sister again to avoid dying is very neat, though the seemingly random placement feels a bit punitive sometimes!
There's one instance were I could not find her on the map!
I think when the two ships become un-linked, your shooting rate could slow down, but your sister's ship could shoot as well. This would compensate your vulnerability somewhat, and also help you find her on the screen more quickly.