But she let the valuers in the house, right? She didn't refuse entry. You can't make her like it, but it sounds like she does actually understand that the sale is going ahead.
YANBU. She can't afford to maintain the place on her income; she'll obviously have bigger bills on the house than she would have on a smaller place. Who's she going to be calling on for help when she can't afford repairs on the house? And that's before we even get onto old age. Suppose she needs to fund care later on, and she's still in the house? Massive potential mess.
Downsizing is the best thing for her as well as for you and db. If she can have a little place of her own, owned outright, before retirement, that's a good result - better than either renting or struggling on in a house she can't keep up. But property prices are unlikely to go down and the mortgage thing will get harder for her with every passing year. If it's going to be possible for her to get that flat, she should start looking soon. She needs a sustainable, realistic plan for her future - "I stay here in a house that's too big and pretend two other people don't own it" isn't it.
She's got very used to being looked after. If you let her, she'll try to get you and / or db to step into the "parent" role. Don't. For her good as well as yours. "I've always sponged off someone else and so I should be allowed to carry on doing so" isn't an argument. Time for her to grow up.
If db is happy to let her have 40% and so are you, seeing as she's buying alone and later in life and is less well off than you, that would be generous. And handy for a bit of leverage, perhaps - say you will sign over the extra money IF she co-operates with the sale and doesn't make this difficult. Anything else you could throw in? Help with moving costs? And lots of help with clearing the house, packing etc. Try to get her excited about it - her own home, where she can pick all the decor and everything and have it all just as she wants it, for the first time ever. Peace, independence, privacy. (There's a lot of middle aged women salivating at the thought...). Offer to help her paint or whatever. Be positive, helpful, jolly about it all. But make it clear that sticking her head in the sand is not an option. She has to face up to things and make a plan for living the life she actually has, on the money she actually has. Get db on board. United front. You're both going to support her in moving forward; you're not going to support her in sitting still. Then you're going to buy her a nice housewarming meal, with gifts that will look great in her new home. It's not a tragedy, it's a new chapter. In a year or two she'll realize she's actually better off in her cosy flat with its smaller bills. Don't apologize for not pandering to her and babying her. Make it clear she will get a lot of love and support from you and db, but only if she acts rationally. She can leave the house and find a nice new place with lots of support, or she can leave the house under duress, managing everything herself and probably with a large legal bill. Those are her choices, and sulking won't change it. Stand firm.