I'm going to really try and not be a "basher" here. So please take what I write with a grain of salt. First sharing a room was awesome with my sister (who just happens to be my half sister by blood but my SISTER). Not having a child because of having to share a room is yes a bit crazy sounding, but I do get it.
As a person who has no "full blooded siblings", I don't understand that logic. My siblines step and halves are my siblings. I love each of them as if we all were from the same set of parents. I don't see them has halves or wholes. It's hard when those around you don't get it...especially parents who always make the distinction between the terms. Please don't make having another child about blood, you will alienate the older child, whether you mean to or not. I am one of a the lucky few who has "other parents" who have always treated me and loved me as if I was their own flesh and blood. However, I do have one set of grandparents that treat me different, and it's quite uncomfortable at family functions. Tread lightly please.
Again I'm not trying to say you are doing this, I don't know your family dynamics but I have to take up for the kids in the same situation I was in.