So sorry to read this OP my mum sounds very similar in many ways.
She favours my younger brother, sister, niece (sisters daughter) and brothers dog and nieces dogs over me, my children, husband and our dog.
She wouldn’t entertain the idea of counselling and would say its all in my mind, she had a feeling I felt like that, didn’t know where I get that from etc etc etc.
Her final kick in the teeth for me will be when her will is read out) I have no idea whats in it but can well imagine).
Brother lives with my mum, sister and niece are falling over themselves to visit my mum. If I offer to take her out for the day or visit I get knocked back as usually they got in there first or she can’t be bothered or will just leave it or will get back to me. I try to visit once a week now but don’t phone as often as I did and I am careful what I tell her. I am pulling back for self preservation. Like your mum my mum also thinks we earn masses more than we do.
We live in a better area than my sister but probably earn a similar amount to my sister who is always pleading hard up and her and her partner are always spending on massive TV’s, latest phones, outdoor clothing, canoes and electric bikes etc.
Like your mother mine doesn’t see anything wrong and never sees the unfairness in how much time she spent with niece looking after her growing up to help my sister out, prioritising nieces feelings over everyone else’s at my fathers funeral, money she spends on gifts for niece, she even gave my nieces boyfriend my dads tools and I have a son, my childrens photos have been put away readon being they have grown up now and the only photo in my mums living room is of my sisters family when niece went off to Uni our wedding photo (sister isn’t married) and my graduation photo have been relegated to the hall).
Now my kids are late teens I never pressurised them to visit as my mother as she did nothing for them and I know whatever I or my family do it would never be enough. I usually visit on my own at a time convenient to me.