HELP! kittens are fighting

hello everyone, i'm not sure if this is the correct forum to post this question, but here goes. i was fostering 4 kittens, and returned them to the shelter yesterday because they were old enough to get adopted. however, one of the kittens had an unidentifiable problem around her nose and mouth(they think it is mange but are not sure), they gave her some medicine, and i have to keep her 2 more weeks and make sure it doesn't get worse. they also gave me another kitten to foster so she wouldn't get lonely. this kitten is about half her size and probably about 4 weeks old. anyways, the problem i am having is that the incumbent kitten is not being very hospitable to the new kitten. the first day, every time the new kitten approached her she would growl and hiss, scaring away the new kitten. today was a little better initially as they played together a little bit, however, this friendliness became shortlived as she has gone back to growling and hissing, and if she is playing with the new kitten, it seems to be incredibly rough compared to normal kitten play. also she will begin to growl if she senses she is losing. it is obvious she is competing with the new kitten for my attention as well, and i worry she gets jealous when she sees me playing with and holding the new kitten and resents the new kitten even more, although i still heap plenty of attention on her. i have tried petting her and attempting to separate them whenever she starts acting agitated. i'm not sure how the new kitten feels about all of this, but it appears he is just happy to be in a home and constantly tries playing with her even though she roughhouses with him all the time and obviously would rather play alone than with him. i was wondering if there was anything i could do to get the incumbent kitten to accept the new kitten, or if i should just return the new kitten back to the shelter and look after the incumbent by herself? the incumbent used to be very sweet and gentle, but it feels like now she is becoming angry and rough, and not happy at all with the new situation i have put her in. thanks!