She isn't your child. She isn't a continuation of your legacy into the future. She and you are components of a family, but not all the components; a single generation does not a family make.
It doesn't mean that what you and your wife have isn't a valuable and precious thing. A good marriage makes each other a complete person. But it is, nevertheless, a different thing. It is a different social construct, to fill a different, but connected, social need.
A marriage is a social tool to deal with the immediate problems of the here and now, preferably for the duration of a lifetime. A family is a social tool to deal with the future and connect with the past. A family can go on long after the married couple have passed on.
I maintain that my wife is family. Baby makes three. This needless hair-splitting leads to the absurd situation in which parents that tragically lose their only child get demoted to "childless couple." They're no longer a family? Bunk, I say.