AUNTIESHELL

I have a 4 year old niece who will be 5 in january,and i have been having so many problems with her,her first problem is she cries about everything,and when i say everything i mean everything,i tell her to wake up she cries,i tell her to brush her teeth or get dressed she cries,now she dont do it all the time she picks and chooses when she wants too cry.when it come to trying to teach her something she cries,she is in pre-k and i will get her to do her homework and she bursts out in this crying thing,where she sheds no tears,no tears ever.if i ask her to write a straight line she'll make it crooked if i tell her to write her name she'll start crying.now her 6 year old sister on the other hand she is like a sponge she wants to know everything and picks up everything and remembers and learns so easy and so willingly. I have no problem with her.but with the younger one is a hand full,now she is very smart,she can tell you somethings that will make you question her age,lol she can recite a whole song,but when it comes to things like being told what to do she freaks and starts the crying thing,she has been to many doctors,where they just didnt seem to know how to deal with kids period,one told me to give them benadryl when they were having problems sleeping and the other one was at kennedy krieger hospital which is for children she told me to just let them do what they wanted as long as they werent hurting each other,now i have had custody of them for two years now,they're mother which is my niece gave them up to social services,cause she didnt want them any more,she use to come see them but i had to stop that because she was coming in and out,she would come and call everyday for say a month and then for two or more she wouldn't,she would promise them things or that she would come and never show, so i had to put my foot down and tell her that until she can start coming and seeing or calling more often then she couldnt see them,oh and she lives 10 minutes down the street from me.so i dont know if maybe all of that and the fact that my niece use to treat the younger one the one with all the problems like crap,she would tell people that wasnt her kid but her niece,she treats the older one better all the time in front of the youngest.so that and cause she comes in and out of their lives is why i stopped her from coming around.so now ive tried everything i could,with the youngest their was even a therapist that use to come to the house every week and she even tried everything she could think of,she even asked me one time have i tried beating her butt,i did once and still nothing,i dont know what to know,please if you have any advice please help me!!!!! Thank you