You have no problem telling little Johnny his siblings were murdered because mommy and daddy found out Johnny would be born? Wow.
I don't believe a body can have a soul associated with it, until there is an interface between the two, i.e., a brain. Therefore, I don't believe the zygote becomes a person until there is, at least, brain activity.
Therefore, those other zygotes weren't siblings. They were other potential bodies that Johnny and his siblings and/or cousins could have inhabited, if different choices had been made.
Now, you want to tell Johnny that he shouldn't even had the body in the first place....
Like that woman in New York that found out she was having triplets so she had two of them aborted.