I don't think there is any reason to feel guilty about no longer bailing out someone who's pissed through huge amounts of money through reckless spending and poor decisions, even when it is clear that the money was running out.
She must have just assumed that you and DSis will continue to bankroll her lifestyle. But why should you when she's acting like a young adult living at home and blowing all her money on fun and trinkets instead of being a responsible adult and prioritising essential rent and bills?
Unless you genuinely have lots of spare money and are financially secure, including where you want to be with pension, housing etc, then tell her that due to the high cost of living in the UK and the failure of wages to keep up with price rises, you are sorry but you can't give her any more money.
You say your DM helps your DSis a lot, could she move in with her? Would that be mutually agreeable?
Otherwise, has she applied for any support she is entitled to where she lives? Does she have a pension? Can she work? You don't say how old she is, which could be anything from late 40s up to much older. Can she sell any of the what sounds like huge amounts of clothes etc that she must have?
What is the normal expectation about supporting older people where she lives? I know that in some countries there is no pension as such and people live with their adult children if they can't afford to live independently.