For a bit of background, DM has been with her partner for about ten years now and they have DM's mother (my GM) living with them. GM is over 90 but in pretty good shape. She can dress and bathe herself, she can make her own cups of tea and cuppa soups. She is continent and mobile and able to go up and down stairs without assistance, so while having her live with them is a source of stress, she's not a high-needs pensioner. Her eyesight, hearing and short-term memory are poor but she still has her sense of humour and she is a sweet and lovely woman.
DM works full time and her partner "Bill" works as a sole trader so his hours are more flexible. He isn't GM's main carer but he does a lot of the running around she needs (e.g. being dropped off at a social day-care she attends once a week, and picking up her hearing aid and taking her to doctor's, for example.
DM is quite open about the fact that she is not cut out to be a carer. Fine, not everyone is. She relies heavily on her partner, but how they arrange care is up to them.
However, DM has taken to using a nasty tone with both her mother and with Bill. For example, if GM is anxiously checking around her chair, looking for her handbag (she has poor short-term memory and frets about leaving it somewhere) DM will bark "NOW what are you looking for?! You leave that alone, you!" and GM is cowed.
If Bill says something to contradict DM she gives him short shrift. I was on the phone to her recently and she suddenly, mid-phone call went ballistic. It took a second for me to realise she was shouting at Bill, not me, because he was concerned she had marks on her jacket. She turned on him with ferocity so I said I'd ring her back. Now was obviously not a good time.
Then I felt bad that I'd left Bill to take the full flak of her anger. Like I'm turning a blind eye. SHe has also left me an extremely angry voicemail. Her anger was not direct at me, but at her phone not working properly (she didn't realise she was leaving me a voicemail. Her rage and outburst were shocking, though).
DH is quite distressed by how she talks to GM (her own mother) and we've had a word. We also offered to look after GM so she and Bill can go on holiday. They kept refusing but last summer took us up on the offer. We thought it might be the stress of having an aged parent to look after, but things have not improved.
How do I broach this with her? She'll get really defensive and I'm not convinced she'd change. She is never aggressive or sharp with me. Only Bill and her mother, neither of whom seem to stand up to her.