Hissing is normal. At least they didn't fight!

Maybe put the new baby in a separate room if you can. Let them sniff under the door...and give each cat something that the other has been sleeping on, so they get used to the scent.

I think two days is a little too soon. But they will work it out on their own, it should be better after 3 or 4 days.

hugs
Catty1