My MIL vacilitates from being competent to acting like a spoiled three year old. Why is this?

Is it for real? She is 76 and has a Parkinson type of disorder. She needs to use a walker to get around. My question revolves round her state of mind. She is depressed, that is a given. She has a home health aide for 12 hours a day. The home health aide "makes" her do for herself, like get up from a chair, go to the bathroom. When there is another party present, ie. myself, her son, a friend, she becomes incapable. She was at our house for a couple of nights and was totally helpless. She fell out of the bed; slid off the couch; was unable to get up from a seated position; couldn't go to the bathroom by herself...ie, I had to pull her pants down, help her onto/off the bowl, and put her back together. On Thanksgiving, we went to a family friends house, and she had a new audience and became incapable. Now, she can hold a conversation; knows left from right; can get belligerent; she "forgets" how to walk; how to sit down, get up, and other motor skills. She also goes into lala land around the health aide, but when I see her, she seems basically "with it", although she will ask irrelevant questions about something off topic, or something stupid like why are wheels round. That all being said, I can't help but wonder if part of this is an act...I have to be careful, since I grew up with hypochondriacs, I have a hard time sympathizing...that got tapped out a long time ago. I am more than willing to assist her in any way necessary, but need to know that she needs the help in actuality and is not just acting helpless and like a spoiled brat. We wonder about dementia, but she is coherent, IMHO when she wants to be. So I am throwing this out here to get advice and opinions from other caretakers to figure out if I have to be more patient. Thank you for your help! Deb