Whether a person suffering from amnesia is considered Mukallaf or not

Q: My mother-in-law is very old and incapacitated. She suffers from diabetes and urinary incontinence. Her clothes and the place where she performs Salah (Prayer) are always soiled by urine. Note that her Muslim maidservant always looks after her and cleans her clothes and room. She always complains that we leave her alone. We cannot sit with her to eat or drink in her room due to the unpleasant odor of urine, so we have the maidservant eat with her. Her daughter-in-law dislikes sitting with her in this room. The children are very young and do not like to be left with her, as they prefer to go out to play. In addition, she has an impaired memory and I usually inform her when Salah is due. However, she performs it when she likes. Sometimes, she does not perform Salah unless we remind her. At other times, we remind her (Part No. 5; Page No. 27) but she claims that she has already performed it. Again, we may tell her that the proper time of Salah is not due yet, but she pays no attention to us! Although we constantly remind her of the times of prayer, she sometimes claims we do not. She then becomes angry and invokes Allah against us. She may also perform prayers together. For example, she may join two Prayers such as Maghrib (Sunset) and `Isha' (Night) Prayers together. Furthermore, she wants us to stay with her all the time and neglect our work; otherwise, she gets upset and invokes Allah against us. She fasts in Ramadan, but by the end of the day before the time of Maghrib, she asks for food to break her fast. She gets upset if we do not respond. Due to her old age, she suffers from hearing impairment that makes it difficult to persuade and teach her these things.Although her son is very busy, he visits her every morning and evening. What is the ruling on the Salah and fasting she observes? Are we sinful for not staying with her? What about her invoking Allah against us? Please, advise. May Allah reward you!