ok, point taken, some sibling HATE each other.....
but
you are assuming that if and when your DD agrees you will just be able to have one, and life doesn't work like that.
what if tomorrow she decides YES, I would like a baby sister and you try, then you fall for a brother, and she decides she doesn't want either......what then????
children are fickle....they change their minds constantly.....plus also she will only see the fun side possibly, and will not have the maturity to know the hard side of it all.
here is no right way and no wrong way, but you asked for an opinion, and some of have stated them.
families are what you make them, and yes, children are part of families and so should be counted.....but she is not the one falling pregnant or caring for the new baby, YOU ARE, and in all honestly, it should be you and DH making the choice and then allowing her to experience the joy of it all from then on.