Mum is so nasty to us but so sweet to her friends. Any advice?

I’ve never spoken about this to anybody before, even within the family it doesn’t get discussed, just brushed under the mat. There’s something wrong with Mum, I have thought for a while it was bipolar or split personality disorder as she can change in a second. It’s quite scary to witness, one minute she’s hysterically shrieking with laughter to a neighbour on the doorstep or phone and 2 seconds after they’ve gone or hung up it’s like a big black cloud descends and you know to leave her alone! It’s as if she hates me and my Dad. We walk around on eggshells around her, scared of her, this has gone on for as long as I can remember.
I’ve taken some advice I read on hear about documenting events when Mum’s in a particularly bad way. I have several videos of her screaming, pacing around the house and muttering that she wishes we would die of cancer, I’ve also been keeping notes in a book. I’ve found in Mum’s case a ‘bad’ episode will be preceded by a good episode. It’s almost like that saying, ”if you think it’s too good to be true.. it probably is.”
Like when she invited my sister and her family to stay for Christmas after years of them avoiding visits because of her behaviour, (telling her then 5 year old grandson to “F-off” at Boxing Day dinner) only for her to change her mind within a week and un invite them.
I reached out to my Aunty who lives halfway across the world and was very much let down by her. She rang and told her everything, (speaker phone) even though everything I’d said to my Aunt was true and I only asked if something had happened in their childhood around Christmas time to trigger Mum’s behaviour, like did a traumatic event occur? Apparently not, and anyway, it all got turned around and my siblings and I came out of it seeming like ungrateful brats. Whatever my Aunt’s going through she wrongly compared and consoled, don’t worry Sis, I’ve got one of those at home to suffer with too, I know exactly how you feel. Except she doesn’t and they’re not alike in the least, bubbly Aunt versus depressed and emotionally abusive Mother.
A couple of years ago I bought a SAD light box machine for her, she never even tried it, it went in the bin.
Dad has doted on Mum from the age of 17, lavishing gifts such as expensive jewelry, steiff bears and holidays on her, but even he is getting to the end of his rope, with his own health deteriorating at an alarming rate. Just before Christmas he stood up to her and told her he was cancelling their two holidays this year and the Rolex watch she’s never worn will be sold to help pay off the mortgage, I was really shocked but proud. He’s not done either thing he said he would.
Mum doesn’t drive, she never passed her driving test but we live in a small village about half an hours drive from anywhere, she works 2 mornings a week down the local shop for 5 hours at a time and walks the 250 yards up the hill to the house, there is always a cup of tea and her lunch ready waiting for her at 13.30 on the dot lovingly prepared by Dad. This drives me crazy, and I’ve told him numerous times to stop waiting on her hand and foot because she never does anything like that for him. In fact she waits for him to go to bed then eats packets and packets of biscuits and chocs. She doesn’t get how good she’s got it but complains constantly that Dad’s an alcoholic (not true) and a junky (also not true, it’s his prescribed medication) she even rang my Dad’s own Mother who is 86, has COPD, Addisons and various other health issues to tell her how awful he treats her and how horrible her only son is. Luckily my Nan is a tough lady and took everything calmly with a pinch of salt.
Im so worried I’m going to end up like Mum and this is hereditary. Her own Mother became a recluse refusing to answer the telephone or front door, she died on her own after an untreated cancer illness.
I very nearly got away from her a few months ago but with the virus, I lost my job & deposit on my first house. I feel utterly trapped.