It sounds as if your kitten is very frightened and lonely, and is calling for its mum and littermates. (Since they haven’t shown up, the kitten thinks it just needs to call louder and longer.)
Since Izzy is behaving so well, I wouldn’t keep Izzy away from the kitten. She may turn out to be crucial to the kitten adjusting.
The kitten is exhibiting a fear response to nearly everything, according to the amount of hissing you’ve mentioned. Keep calm and patient. Try tempting the kitten with particularly enticing food – a saucer with a tablespoon of plain, unsweetened yogurt, perhaps. Set it on the floor, near a wall; then sit on the floor yourself, at least 4 feet away. Maybe bring a book with you. The idea is to have you not handle the kitten, hold or pick up the kitten… the idea is to have the kitten learn to associate you with something good (tasty food), while not being so frightened by your presence that it won’t come out for the food. So you need to make sure you’re out of arm’s reach, initially.
If you want, Izzy can come in, too, and lick up some yogurt (in which case, make sure you have more to put down). The kitten will be bound to observe Izzy eating the yogurt, but probably won’t approach as long as Izzy is near the dish.
Kittens who are very fearful are simply showing an instinctive behavior that normally, in an outdoor, wild situation, would keep them alive beyond weaning age. Sometimes kittens who are very fearful have already had scary experiences. Please be as patient as you can, and try to remember your kitten is scared of you, its new surroundings, its new kitty companion Izzy, and so forth. The kitten will need to develop trust on its own timetable, and that’s usually a bit longer with kittens who are frightened.
Good luck!