How would a share of a house ever get to the children of the deceased partner if the other stayed in the house?

[quillfram]

Hi!
My mum passed away recently, and my dad has just bought a house with his new girlfriend.
They plan on marrying in the new year.
He said that he is protecting our interests by buying a large house. He has a mirror will with her (I don’t understand what this is really, but as far as I can tell, this is basically like having no will at all as she can just change it without him knowing? Why on earth would someone choose a mirror will over something else?)

He said that its in the will that if he passed away, she has the right to stay in the house, but if she sells it then she gets 50% of the value and me and my sibling get 50% between us.
But if she is the surviving partner, how would we even know if she came to sell it, if we have nothing to do with her after my dad dies?
How would his wishes be honoured if she stays in the house, then dies? Or goes into care. Surely her grown up kids would just sell the house and share the value between them?
We wouldn’t even know it had been sold most likely.
Can anyone advise on what would happen? Would us getting our share just be at her/her kids disgression? Would we have to check in every few years or something?
Does this arrangement actually protect nothing at all?
Where would we stand legally? Anywhere?

I’m not here for the lectures on ‘no one is guaranteed an inheritance’ etc - its my late mum’s money he’s bought this house with, its about honouring what she would’ve wanted to happen with everything she worked for.

thanks.

Have you actually seen the will? If it’s definitely tenants in common and his 50% has definitely been willed to you then on his death you’ll own that 50% (whether she’s allowed to live there or not wouldn’t change the fact that you own 50%). She wouldn’t be able to sell the house without your permission, it’s not fully hers to sell surely? I assume the solicitors check this stuff when you sell a house - that you actually own the house you are trying to sell.

See if she also mentioned her children in this new Will as beneficiaries for her half of the house regarding tenants in common. In the event of her death.

She doesn't sound very trustworthy