Join Date: May 2008
Location: St. Albert, AB, Canada
I'd bring the new baby in in a carrier and set her in the middle of the floor just like that. See how they react to each other; excited, scared, upset, ect. A little bit of hissing is fine, but watch the rest of the body language.
TBH I've done straight intros at that age and it's always gone nice and easy. Use toys and treats to break the tension since kittens at that age have short attention spans. They'll likely be best buddies within a few hours.
That being said I wouldn't leave the new kitten alone with the older one, but that's mostly because at that age I'm all about promoting good litter box habits by putting the young kitten in a smaller room with a litter box when they can't be supervised.