I don't think at this age it would make much of a difference. Think it would probably make more of a difference if a baby were added because your daughter is so young and might be harder to adjust. (not to say that's a bad thing...in general siblings are added to the family this way all the time!
)
Plus, with the age and sex difference, there will be less issues in general. (toy sharing, room sharing etc.)
How sad for the little boy and hope regardless who adopts him, he receives all the adjustment & grief help etc. he needs. I also hope it's a slow transition for both him and his mom giving all of you time to adjust.
Crick