I'm sorry to say it doesn't work that way. Your husband should not be expected to focus less on any of his kids just because another one is born. Blended families come with many challenges, and this is a big one. But you already knew he was a package deal, kids and all, when you married him. If you didn't want to have to put up with stepkids then you should not have married a guy who already had kids from a prior marriage.
My suggestion is that you try to be more understanding of your stepkid whose world has been totally up ended as a result of the new step mom and new step sibling and who likely just needs some time to adjust and a little compassion and understanding for the emotional roller coaster all of this has caused.