Frankly I think two seperate family zoom meetings are in order. First one, without DM. Starting point of that meeting would be to share info on what everyone knows/thinks is the real situation re the needs of the works to the property. Probably this will end up as a list of questions no one yet knows the answers too. Scope a few scenarios, being clear it’s an info-gathering exercise only. Likely this is where everyone gets a chance to outline what their household can potentially afford to give in ££ for any works, and what assurances/participation they’d want in the works, and the eventual equity of the property.
Then a delegation talks to DM. This will likely be tricky- sounds like she’s been ‘in her own head’ about the problems a bit too much, and also has fixed on a solution that she’ll need to be talked (slightly) out of.
Whatever ends up happening, it probably isn’t going to go as she imagines at the moment. That’s ok, even if she has a defensive reaction when you say this. She’ll mull it over, and see that if she needs other people involved she needs to include their perspectives as well. Impress upon her that this is a project, and will require more time and energy to plan (as a group) than she hoped. The money conversation is not the first one.
Next identify any really urgent issues- is she freezing? Is there rain coming in? Address those, and quickly. Start with her savings for this. Prioritise any other works, and assess if there are better suppliers of those trades, at better prices. Basically, take the project out of her hands (all of you) as a team.
During these steps, figure out the true state of her finances. Start talking to her about the future, possible care scenarios, etc. Introduce the idea of equity release (we call it ‘reverse mortgage’ here). Help her slowly to see it all from a broader and more long-term perspective. ‘It‘ll be your inheritance one day’ is hardly the full story, or a useful line to take at this point. Much as it’s easier to think we’ll all go from spritely to ‘passed away in my sleep’, it’s hardly ever what happens, is it?
I always say to my family elders ‘If you care about your independence, then define what you mean by it clearly, and plan for it. Crossed fingers and denial just lead to a quicker, and more complete loss of your independence.’ It has worked for me in the past to take this line. Good luck!