My 84 year old mom refuses to stop sneak feeding our 7 year old cat who is pre-diabetic. Any suggestions?

From the day we told my 84 year old mother the vet said we need to help our cat lose weight, my mother has refused to be on board with this. She says she had a cat for 17 years and she won't accept the vet's advice. It has been FOUR LONG YEARS. She stays up late and sneak feeds him (on the counter), hides food in little dishes in closets and drawers for 'later' when we go to bed, gotten out of a sick bed to sneak feed him part of a Subway sandwich. I got so upset I hyperventilated. I've begged, yelled, deliberated, reasoned - she will not stop. She told my husband, 'you don't love him anyway' and this was very upsetting to my husband. We are at the end of our rope. This has fractured our household. She doesn't take care of anything to do with them. She sleeps in till Noon - and stays up later. We get up a 6 and to bed by 8 or 9. I've found dishes in her room under her dressers. It goes on and on. I freeze all leftovers immediately, do not get lunchmeats - and if we do - we have to disguise and hide them. Hidden the cat food in containers up above the refrigerator where she can't reach. Chinese food is a total PANIC - I had to freeze leftovers cause she would give him shrimp. My husband is so angry he can't even deal with it anymore. I am totally panicked and the anxiety level is off the charts. I even hear her getting up and I get adrenalin and dread. She doesn't CARE. She says 'who is HE to tell me'. I believe now she is getting back at US and the cat is the sufferer - cause she just wants to be stubborn and nobody will tell her ANYTHING. I have taken care of her since 1982 when my father passed away at 54. I have never realized this side of her - she is failing in memory and I started her on meds, she uses the memory thing and lies she doesn't remember. I can usually tell when she is - but it makes everything so difficult cause you don't want to accuse a person of something cause half the time she doesn't REMEMBER she hid the food....She is having bowel issues and lost them all over the kitchen floor - STOOD THERE and told my husband she was wiping up a water spill. I went in and she had a POOP TURD on her SHOE. She REFUSES to tell us she needs help cause she's so PRIDEFUL. It is a constant GUESSING GAME to care for her cause she HIDES EVERYTHING. It took hours and hours and days and days to transition her to the cane and then the rollator. She doesn't have all her faculties, however, with my husband she has a STRONG MEAN side and no problems...she needs me so she curbs it and shows me her weepy side. She believes it has to be her and me and she resents him. He resents her for her 'using' and 'manipulating' me. I answer to God and will always do what is right - however - I cannot support her misguided behavior. Every time I am sick (IBS/heart SVT's) she creates a problem and they end up in a verbal battle. She has hit my husband in the arm, accused him ot keeping me prisoner in our bedroom, and yelled 'don't listen to him, he's teaching you wrong things'. The clincher? I'm 58 years old and my husband is 60!!!!!!!! He was taking medication for a blood disorder for a YEAR and she still taunted his emotions and upset him over and over even though I begged her not to cause it was bad for his getting well. She is selfish and narcistic it seems and I'm still reeling to realize this awful side of her. My dad was a minister and she was a minister's wife and my husband says she doesn't act religious. I can't defend her. We are going to talk to the preacher. My husband says she need a psychiatrist. I don't want to make the wrong decision here. My mother is a loner and I believe now I've shielded her from much - but I did not 'make her' personality to be like it is now. I am not willing to sacrifice my husband or the cat for her. We are truly at a breaking point. I have 2 sisters who use her for money (their whole lives) but have no relationship at all. I am the middle daughter and the only family person she will ever and has ever had to care for her. She threatens in angers that she wants to go to a home, yet cries and is scared at the thought. She is really a scared negative person who has low self esteem. I will never have her leave this house because I know she would be miserable - yet feel I am going to implode with the stress. We have a senior place also and we talked to a counselor. He says we don't need therapy - she does. I thought perhaps she and I could talk to a counselor or someone medical after the pastor who my husband and I are going to talk to. Any suggestions?