Caregiver of a 90 yo lady. She is demanding and manipulative. Any advice?

Knowing her son is upstairs, she starts yelling when she doesn't get her way. She has small irritable bowel syndrome and, I am told, she has Sundowners syndrome. I am constantly getting her up to use the bedside commode, but 75% of the time, she will not use the bathroom. She starts shaking, and becomes confused, but when I ask her questions, she is alert and oriented. She starts yelling for her son to come down when I do not give her water, which makes her want to go constantly. I tried to sit down and explain to her that she needs to limit her fluid intake and by drinking fluids when she is trying to sleep, will only keep her up. I also tried to get her to sit up but the minute I get her up, she is yelling to get back in bed, vice versa. She is nice one minute, the next she is unbearable. I try to talk with her. I ask her if there is anything that is bothering her besides wanting to get up. She just shrugs and says she does not know. She then apologizes for her behavior but five minutes later she is doing the same thing again. I plead with her son to give her sleep aid, but he makes up all types of excuses why she does not need it. I tell him that she needs to sleep. She is agitated from lack of sleep and that she is fixated on getting up and down. I have no idea why he thinks that will make things worse. Her doctors prescribe sleep aid for her for a reason. If I cannot have her son work with me there is little I can do to calm her down. I end up waking him up to come down and talk with her because she will not listen to what I have to say. She starts to cry like I am hurting her when I am getting her up, but I am always careful. I get so frustrated I take time out in the dining area, but the minute I leave she is yelling again. I feel she is manipulating me into having her way which I end up doing. I get so tired from getting her up to the commode every 15-20 minutes. After a while, she starts to get weak and I am dealing with total weight and she is not light. She has fallen several times since I have been working with her. Thank God she has not broken anything yet. I have tried everything from talking with her to getting her up in the wheelchair so she can sit up, but she wants to get back in bed after being up for 5 minutes. The cycle of getting up begins again. I do not want to lose my job from frustration. I need some advice.
I work with her at night from 830pm to 530am. There would be periods of a week when she will sleep calmly and wake up only twice the whole night. Lately she has been up constantly. It is driving me nuts!! When I mention these things to her son, all he says is "I know, I know" and walk off frustrated. It seems like she does it because she knows her son is upstairs sleeping and she can get away with it by yelling for him. I am about to give up on her. I have been with her for 9 months now. The day shift worker says she does the same thing during the day and she has been with her over a year now.
She does become verbally and emotionally manipulative. When she sees my frustration she starts asking me if I am ok and starts apologizing. I tell her I am fine, I am just trying to figure out what she wants me to do. Like I mentioned, all she does at this point is shrug and says that she does not know. I do not want to injure myself or her, so I am in desperate need of some suggestions. I even tried calling the client coordinator and all I get is, "She has Sundowners and that she will get worse" HELP! I have taken care of clients with Dementia and Alzheimer's disease but they are always cooperative when distracted. So I know she knows what she is doing and will not stop once she gets started.
Anyone with suggestions?