I know 'some' of what you're going through, as I lost my husband to cancer some years ago. But I didn't have a terminally ill mother with severe depression to cope with at the same time....
I'm afraid I agree with the others - it is your husband/partner who is the priority, and not your mum, and he needs your focus.
So, I would say two things are possible.
IF you and he have a home of your own, and he is physically capable of it, then go back home, the two of you, and get all the care you can for him (see more below). Leave your mum in her own property, alert SS you are withdrawing care apart from, say, an occasional visit (eg, if your partner is asleep or whatever)(not sure how far away your own home is of course). Either she will cope, as she says she can, with carers coming in, or she won't, in which case the SS will probably just move her into residential care or a hospice anyway. BUT, most importantly of all YOU STAY OUT OF IT!
However, if your partner can't really cope now with moving back to your own home, then stay on in the flat at your mum's property, but, again, WITHDRAW YOUR CARE FOR HER!
The essential bit is you NOT providing ANY of her care (other than occasional social visits - NOTHING ELSE).
On absolutely NO ACCOUNT continue with her care and on even more absolutely NO ACCOUNT allow her to separate you from your partner.
It's essential you now devote yourself to him, for whatever time he has left.
As for your mum, she must do without you. Yes, I guess there's a chance she may possibly pre-decease your partner (I would hope so, in a way, as in, I hope he survives a lot longer than she does). You say she has bone cancer, but is this a primary bone cancer (very rare in adults, so I understand??), or is it from a primary elsewhere (eg, breast cancer that was successfully treated some years ago) that has now resurfaced as bone cancer (ie, secondary cancer).
Whichever it is (and, I believe that secondary bone cancer can be slow growing, hence the longer survival times she's had than originally thought), I would ask her GP outright what her likely life expectancy is now. Is she still having 'anti-cancer' treatment for it, ie, drugs or radiation etc etc that reduces the amount of cancer in her body, or is she on 'palliative only' treatment, ie, to keep the symptoms/pain at bay? (This will give you a clue as to what the doctors think of her life expectancy)
More importantly, of course, how is your partner doing? As you probably bitterly know by now, pancreatic cancer is one of the most dangerous of all cancers because it is nearly always found too late for curative treatment (the even more bitter irony is that, found early - usually by chance - it can be very curable/treatable......)(true of most cancers alas.).
I hope you are checking out the various pancreatic cancer support groups and charities etc, to find out what the latest treatments are (remember, new drugs for cancer come on stream all the time, though many, shamefully, are not available on the NHS - they may not cure, but they extend life, sometimes by a considerable margin over the doctor's prognosis). You may have to PUSH HARD to get hold of those treatments, and ensure your partner is with an oncologist prepared to treat his cancer aggressively (some advise 'go home and make your peace' etc)(which, yes, sometimes, is really the only option left - and yes, some patients chose that course because they don't want to go through unpleasant treatment that is difficult to endure, rather than make the most of what time is left to them.)
Finally, please do get in touch with your local hospice if you have not already done so. Hospice-at-home care may be available (it was for my husband), and the hospice nurses are usually wonderful. Good 'end-of-life' care can make a heartbreaking situation just that little less agonising to bear.
I do feel for you - this is an unbearable situation for you, but I do most heartedfeltedly believe that this time you have now with your partner must take priority, and that it is a time that will tear you to pieces.....but light the rest of your life with precious, precious memory. Make the most of every moment you have with him. Even when he's asleep, or 'out of it'. I used to sit beside my husband as he lay on his side of our bed, and hold his hand, and read, and have a cup of tea, and just 'be with him' as he slid towards the end, day after day......
It was heartreaking, but I poured my love into him, and he was never, never without me or one of his family.
(And speaking of families, your sister is a piece of work, and to be honest, I would write her out of your life - if she can't rally round to support YOU at this time, let alone her mother, she isn't worth a brass farthing!)