The main thing with kittens as everyone said is patience they do all work at their own pace. I think I was lucky having the two this time around because if I just had my boy on his own I don't think he would have come out from under the bed for weeks, my little girl is extremely confident and I think she encouraged him but he's only now coming up on his one for cuddles and spending time with me without her over the past week or so and I've had them for 7 weeks now.
If your girl isn't interested in most treats try getting down thrive chicken treats (it's just freeze dried chicken)
https://thrivepetfoods.com/thriver-100- ... cat-treats they seem to work with even the fussiest cats, Arya won't take any other treats but she really likes thrive and goes crazy when she smells them. I think some supermarkets stock them now.
For toys to try and get her to interact as said above wand toys tend to be the best usually I've found anything with feathers is a winner, you can buy a da bird over here from Amazon.co.uk also the store linked below is great for these types of toys in the uk at decent prices.
http://www.purrsinourhearts.co.uk/shop/best-cat-toysI would go with getting either a Feliway plugin or just one of the travel sprays and spray some areas of the room where she hides so she feels safe, really all Feliway does is help with the transition and keep them a lot calmer and helps with her separation anxiety from mum and her siblings. The other thing that can be useful is a catnip spray and just spray some toys as that can help encourage her to play. The best thing you can do is sit in the middle of the room and let her come out to you and let her sniff you for now refrain from stroking too much or picking her up etc. What you need to be is on her level so try and stay low to the ground with her so you seem less threatening.
It can take time with my girl Mina it took months for her to adjust and stop being frightened all the time, all you can do is take it slow but definitely get some Feliway and some thrive treats and they should help encourage her and I have noticed with kittens on their own they take longer to adjust sometimes because they miss their siblings so much.
If she's still crying for her siblings get her a Petstages Kitty Cuddle Pal, if you can't find the kitty one the puppy one is fine also. Or you could get a kitten heated cushion the one I got below for my two although I haven't needed to use it much as they have each other
http://www.specially4cats.co.uk/cat-acc ... shion.htmlIt's a toy you heat up and give to her to cuddle while she sleeps just to help her settle more.
Good luck and I'm sure she will be fine in no time