It is very hard for your aunt to predict the value of her house and the amount of her cash assets on the date of her death which is where the problem lies.
Daughter A lives at home and will as your aunt ages probably take on more and more of a caring role, cooking, cleaning, shopping, doctor and hospital appointments etc., although from what you say above I am now doubting this. So during her lifetime daughter A benefits from 0 housing costs. It seems she has little potential to build savings so maybe has marginal employment if anything.
Daughter B has probably benefited for being educated and now lives independently from her original family. But is a great help to her mother. She has said she is happy to inherit the non house assets of the estate when her mother dies, and happy for her sister to inherit the home.
I think your Aunt should take this at face value, and believe Daughter B.
So in her will
Leave the house and contents to Daughter A
Value house on date of death, leave cash to the value of the house to Daughter B.
If cash assets are less than value of house daughter B gets all cash assets.
Leave the residue of the estate equally to both daughters.
Perhaps your Aunt has some specific pieces, Jewellry, items, etc that have specific sentimental value to Daughter B, she should call these out specifically in the will and give to B.
And sure at the end of the day B may inherit less in absolute value than A and pay more tax but she also is probably very happy that A has a secure home for her old age too.