There is a lot of information involved with raising a species differing from your own! Take heart, there is plenty of information online for you that your burden may be lessened! It is an honor to take part in the rearing of cats, for they are one of the few creatures of this wild earth that have been with us humans since almost the beginning of our history, if not before!
Never forget that we responsible humans have a duty to do our best in the care of these animals since they are at our mercy in most circumstance. Most if not all of homeless felines are due to the irresponsible acts of irresponsible humans, we owe the cats our help! You and I must pick up the slack and perform a responsible act!
In regards to your actual question, at this age they should begin to form alliances with siblings, and they may gang up on one or more of the litter. This one may be acting this out. The alliances may shift as time goes by. Some times boys will gang up on girls, and vice versa. Or the alliances may be non gender specific. and they may act out in other ways. If this kitten is overly aggressive, I am not sure as to the reason. Make sure they are offered plenty of food, and it should be soft. This is the weaning age, so make sure any food you offer them is moist. If you buy dry food, add warm water and wait till it is slightly mushy, they mush it with a fork. their digestive tracts need time to adjust to the new food, as they are coming off a liquid diet.
You may try feeding them more often, as this kitten may be very hungry. Also, since the mother is gone, you must do your best to be the best substitute mother! Have you been rubbing their bums with soft cloth? They need to be stimulated in order to go to the restroom. The mother would do this by licking their pubic areas. Both urination and defecation need assistance! Softly rub there pubic areas with moist cloth to get them to go. At this age they should be trying to use the litter box, but with out a mother it may be delayed. Make sure they are using the restroom!
Also, the mother would be bathing them, it is important for them to learn grooming at this age. Spend a good amount of time petting them in the manor as they may think you are grooming them.
Make sure you handle, that is carefully pick them up, and hold them often. They are quite impressionable now and need to learn that you are a care giver and they are safe in your hands. They will be much more friendly and lovable to you if you handle them with care often at this age.
There is a lot to take in, I can’t imagine I covered it all! Check out the many online sources. Take heart and thank you for doing your best to give these cats a great chance at life and a good home! I have been where you are and felt over whelmed too! Do your best!
Here is a plethora of links!
General kitten information:
lots of reading and raising stray kittens myself!