I had a lot of trouble with the way their relationship was portrayed in the book. I felt the character of Natalie was too one-sided. It felt like her actions prompted by her jealously of Alice were too extreme and therefore not very realistic. I also didn't get a feel for what Natalie was like all the time they were living in Orion. Maybe if there had been a chapter from Natalie's point of view it would have helped.
I also felt the character of Alice could have been a little better developed. I know the pain from a disease like RA can affect one's personality, but I would have liked to see some flashes of the spunk Alice exhibited at the beginning of the book.
I would have liked to see Natalie be kinder to her sister, and Alice be more independent, so I didn't feel a lot of sympathy for either of them. I had very little sympathy for Natalie. Being sterile due to an abortion was sad, but her actions were too cruel. But I did have more sympathy for Alice being separated from her child and told she had died. If she had had a child to raise, she might have had a more positive outlook on life.