I think my kitten is being aggressive

i need some advice , I have a 13 week old female kitten , on Sunday I decided to get her a play mate I thought I was buying a 8 week old male who was crying out all the time & would only settle when I lay him on my lap but after a trip to the vets it turns out he's only 5 weeks old .The first 2 days the female kitten just hissed & growled she wouldn't let him play with any toys use the litter tray even though I'd brought him his own the poor little thing would freeze in the middle of the floor & cry out shaking so I started to tell her off with a loud clap or a smack on the bum I even tired a spray bottle but nothing worked s,I thought she's just marking what's her & making sure he knows she's the alpha . However today the hissing had pretty much stopped & he seems a lot more confidant but now she keeps jumping on his back & biting scratching him & hissing right in his ear he is now trying to stand up to her his the odd his or swat back & I though it was time I left them to just make friends so to speak. He is so small she's 3x the size of him so when she jumps on his back he's stuck on the ground she bites his ears legs & tail while he is screaming out making a howling noise sometimes he will just get back him & follow after her so I assume his okay but other times she continues to jump on his back & the howling gets louder she won't stop until I clap or pick him up its gotten to the point were my 6 yr old daughter has been in floods of tears cause of the noises hes making , It's not a non stop thing she will now let him eat & doesn't try to steal his food or stop him using the litter tray she even lets him sleep sometimes on her lap as you can see in my picture . I'm just wondering if anybody else has had this before & is she trying to hurt him or just simply playing & his a little to small to match her ? They are only together when supervised please help me i don't know wether it was a good idea to bring in a second cat ?

The first thing to check is whether or not they're just playing. This guide:

http://www.thecatsite.com/t/277579/reading-your-cats-body-language

shows you some of the different body language cats use. When my boys wrestle, they look and sound like they're killing each other, but it;s just a game for them. 5 weeks is also really young - too young really to be away from mom and too young to have learned social skills with other cats. It may be that she's just teaching him manners. It could honestly be that they're trying to play but the size difference is still a bit too extreme. I would keep an eye on them and as long as no one's getting hurt or appearing honestly frightened, it my just be a very loud game.

He is was to young to be away from his mother despite the vet telling me his 5 weeks old the lady I brought him from assures me he is 8 weeks old but his mother rejected him & only him at 4 weeks old & his siblings bullied him they refused play & wouldn't let him eat but she wouldn't take him back either way so his with me now . Its as if she thinks he's a toy she bites his neck ears & back legs mainly but 9/10 regardless of the howling he does during this sessions be always goes back for me when I pick him up he wants down again so it can't be hurting me that much I think the size difference just worries me as she will over power him everytime as she is very boystrush They are separated when there's nobody around as I wouldn't risk leaving them alone just incase. I do check them both & either seem to have any obvious injures there's never any blood , he is very vocal he will cry when he can't get the right position using the litter tray or if I pick him his food bowl while his eating I get growled at so maybe he just likes to me Heard I don't know but I'd hate to think I'm just sitting back & watching them go at it if she is really is hurting him although from were I was Sunday to were I am now there is a massive difference she had calmed right down 4 days ago she couldn't even look at him without the hissing , growling & hunching her back now there is non of that at all . Fingers crossed as he gets a little bigger is won't look as bad . Il check out that link thanks

Poor kitten, having been rejected from the litter so young. Thank you for taking him in 

I think you're doing the right thing letting them work it out. They are probably too young to really hurt each other and they will get used to each other. It's usually recommended to introduce cats to each other gradually, but kittens are often an exception.

I'm glad you stopped reprimanding her. As a basic guidelines, never shout at a cat, clap, make loud noises, and definitely no smacking (ever) and no water spraying, when they're interacting with another cat. It doesn't matter if the interaction is positive or negative, when you startle the cat, you make her associate the other kitten with danger and pain and you're actually making the problem worse. 

Here are a few articles which may help - 

 [article="32493"]The Dos And Donts Of Cat Behavior Modification​[/article]  

 [article="32680"]How To Successfully Introduce Cats The Ultimate Guide​[/article]  

 [article="22328"]How To Stop Playtime Aggression In Cats​[/article]  

Hey ,
They are like different cats now there's no more hissing or aggressive behaviour from eitha of them. They play together loads most of the time it's him chasing her around they play tig & hide & seek its like having to young children watching them play they have me laughing all the time . He copy's everything she does his defiantly learning from her. However he still makes them noises when they play together but if I pick he up he makes the noise so I'm beginning to think his just very vocal cause they can play & his making the noises but as soon as she stops he goes straight back for more . I'm still keeping a close eye on them & I will still be separating them when unsupervised . They seem to becoming best friends & that's the best I could have asked for I'm so glad I don't have to pick one to get rid of