You are using the new baby as a means to an end, not valuing it in itself
The saviour sibling will always feel less loved
The donor sibling cannot give its consent
The procedures can be painful and even life-threatening
As the donor sibling grows up, it may decide not to help, but may be forced into it
There are alternatives - before 2000 this wasn't even an option, so why is it needed now?
What if bone marrow donations aren't enough? How far would you go to save the existing child's life?
It feels wrong, which is your conscience telling you it is wrong
You have to accept God's will, even if you don't understand it. Creating a saviour sibling is going against God's will and trying to stop something He has allowed to happen
It goes against the Human Rights of the new child
It doesn't always work, and then how would the parents feel about the new baby?
You are playing God; it's not natural
You are creating a 'designer baby'
Doctors are putting the donor sibling at risk, with no possible benefit to them - this goes against the Hippocratic Oath ("abstain from doing harm")