My feelings are mixed over this.
IF the parents want the other sibling for him or herself, not having the child only because they want to help the ill sibling, then I am heading towards ok with it as long as the well child is not going to be forced to go through procedure after procedure before s/he can be of the age to consent.
The situation in “Her Sister’s Keeper” would in NO way be acceptable on many levels. One of those levels is that the girl was subjected to these extreme procedures before she was of the age to consent or even understand why they were happening.
If the child is being conceived only to help an ill child, I wholly disagree with it. As @Dr_Dredd said, that would be using the child as an object.
As @stranger_in_a_strange_land said, this would be heading into the clone debate.
End all as far as I am concerned: no child should be born for any reason other than as a child him or herself out of love. A child should not be conceived as a tool. If I was a father, which I am not, and had a child with medical illnesses like we are discussing, I would not have another child just to save the first. IF I had another child, it would be because I wanted the other child as him and herself.
If I had two children and one came down with an illness that the other could save, I would be absolutely torn as to whether or not to have procedures done that would cause the other child pain unless the child was of age to give informed consent for such. It would tear me to no ends but, if the child was of age to give informed consent and denied it, I would not force. I would do my best to convince, but if the child said “no” that would be their right.