I've posted about this on mumsnet before, but the situation has changed a bit in the past year.
The back story: dad did equity release on parental home a few years before he died. My mum never fully understood what she was signing her name to - she didn't understand the concept of compound interest, still doesn't.
Five years on from dad's death and mum now (now 81) lives in the family home with my older sister, who raised a 150K interest only mortgage on her salary to pay off the equity release, and is servicing this fairly large mortgage while she lives at home with my mum. If she hadn't done that it is likely the bulk of the value of the house would have been consumed in a fairly short period of time, leaving my mum stuck in a detached house with a massive garden, which she would struggle to maintain on her own, but unable to leave as she wouldn't have enough money available after paying the equity release off to buy another cheaper property in the very expensive area where she is determined to see her days out.
Now here's the problem: dsis has psoriasis which has become very very bad recently, to the point that her hands and feet are almost completely raw, and she is struggling to get through every day in her very strenuous and responsible (and physically active) job. She has to sleep with plastic bags on her hands and feet, comes home every with blood leaking out of the dressings, and has to spend half an hour after work every day debriding and soaking her hands and feet before wrapping them in clingfilm. 
This results in her barely managing to keep on top of her work and suffering from quite intense anxiety about it as a knock on. I can't see how she can continue in her job and neither can she, but if she leaves how will she service the mortgage? She is 53 and would struggle to get a job that in any other sector which would be feasible for her with her health problems. She has had this problem for years and has tried many different treatments which until recently kept it under control enough to allow her to function. In the last year however it's got really out of control.
Anyway, a family friend died earlier this year and left my mum 120K and my sister 30K, enough money to pay off the mortgage. As soon as my mum phoned me and told me about the inheritance I said 'brilliant, now you can pay the mortgage off and you can both stop worrying about your future in the house' (ie, can you afford to stay or will you have to sell and downsize if you can't pay the mortgage). DB said exactly the same, and so did my mum's best friend.
All fine. Except not, as mum has decided that she wants to use her part of the inheritance to get the (perfectly decent and functional) kitchen remodelled, go on cruises, and generally live it up, while my sister carries on servicing the mortgage.
My brother put it to my mum that my sister is really struggling with work, to which my mum's response was 'she's lucky to live here in such a lovely house, she's made her choices, now she has to deal with the consequences'.
I feel gutted and angry with my mum. My sister has grafted all her life and never complained. She has never had anyone who's 'got her back'. Her ex partner of 19 years was profoundly selfish and insisted on separate homes/finances until she left him. Then her next partner offered her the security of a home together, but turned out to be a violent abuser. He was a gambler and an alcoholic who ran through all the equity my sister had when she sold her flat to move in with him, and she ended up back in a rented property at 43 with nothing in the bank. She has never asked my parents for anything and is the most honest, sincere, principled person I know. I feel distressed that my mum can't take her needs into account, given that my sister is struggling so badly with her psoriasis. My mum's quality of life is so good for someone of her age. She has a very comfortable income, is active, is still driving, has enough of an income to eat out several times a week, go on holiday with her friends, employ a cleaner and a part-time gardener, and put money in the bank every week. She has a better social life than me and she hasn't worked full time since she was in her 20's. I could understand her wanting to live it up on her inheritance if she didn't already have a really good quality of life, hadn't already travelled the world several times over, been on a cruise, lived in beautiful homes etc. She's said that she wants to give some of the money to my db and me, but we've said firmly that we don't want it, that we want her to pay off the mortgage with it so that this stops being a worry for her and my sister.
It's like she can't compute that not paying off the mortgage means that my sister is trapped on a treadmill of full-time work which she is becoming too ill to cope with. If I try to get this point across to her she gets angry with me and closes down. Tells me to stop upsetting her, that she's old and can't deal with people upsetting her.
I feel oddly distressed about how hard-faced my mum is being. She's a loving person, but she's not behaving like a loving mother to my sister in relation to this issue. I can't see how it's going to pan out right now, and I'm worried about it causing a serious rift between us.