Any suggestions to be able tell if my mother with borderline tendencies is beginning dementia or if it is mental aspects & manipulation?

I am new to this forum having just found it yesterday when searching for answers. A little history... I am an only child (daughter) and have been dealing with all the issues of my mom's borderline personality and behaviors since childhood. My mom is now 83 and has several health concerns. I live beside her and my stepdad in a duplex (I know ... classic codependant) that we built in 1983 when I got married. My stepdad is 12 years younger than my mom and has been married to her for 5 years. She did pretty good with most of her "issues" for awhile, but about a year ago started showing her old self. Coincidentally it happened when he took a part time job and she hated being alone. That job ended and her physical problems improved some, but she continued with a decline in health. She has become homebound and needs a walker to walk if she is not being assisted by him or me. 3 months ago he went back to work part time and all the emotional issues returned. For a couple of days, she was almost out of control and switched between crying and being furious at being "left alone". Now, that is somewhat better, but she still hates him working even though she knows it is necessary for him financially. She goes to bed when he leaves (about 2:30). She has complained of several health issues (coughing, shortness of breath, weakness, etc) which are all valid concerns since she has a lung disorder and also has had problems walking since 2 hips were replaced 10 years ago. She has fallen several times getting out of bed to go to a bedside potty. Recently she had diarrhea from antibioics prescribed for frequent uti's. She has had bladder issues for years and has to wear incontinent pads and the dr thought the UTI's might be coming from that. Since the diarrehea, she has exhibited more issues...calling me several times in a row (as soon as she is alone) and leaving basically the same message. She asks my step dad what day it is over and over. She can be quick to become irritable if things are not "her way", but in a complete switch...she says "I love you" at the end of almost every conversation. She has days (when he is off) where she doesn't show these things. My husband and friends think she is manipulating me so I will be there when he is away. I can believe this because I have been there all my life. I know I have been "taught" to react to her.

I guess I am trying to figure out what reaction is appropriate. If it is anxiety, control and narcissism (in other words her personality) then shouldn't I respond differently than if she is beginning to have dementia? I am starting to have issues myself due to dealing with this and am desperate for answers.

Sorry this is so long, but I felt like I needed to give some background since this is my first post.