Adult Sibling and Grandchildren Living In Home, Parent Dies

Short answer: yes

Long answer: how much of the value of the house do adult children 1, 2 and 3 want to spend on legal representation?

If the widow is still alive, I suggest getting clarification of her wishes asap.

Both my parents had siblings that never moved out of my grandparents home, both were given right to reside until death and then passed to any surviving siblings.

It was agreed in advance and respected in both cases

Could be war otherwise if there's misunderstanding, siblings will fight to the last penny over wills in some cases

They are licensees of the mother. When she dies and the executor tells them to get out, they will be trespassers. They should have bought a house long ago.

could be tax implications for the non-resident 3, though only if it's a particularly valuable house or there are significant other assets.

If I were one of the non-resident 3, I wouldn't be making any plans for the inheritance money, it could get very messy.

There will be no question of fees being taken from the estate unless there is a challenge to the validity of the will. If the house is split 4 ways and one of the 4 want to say the will was invalid because it failed to make adequate provision for them, there is a possibility the costs could come from the estate. The problem with this is that the house will then have to be sold to discharge the costs.

If there is no challenge to the validity and one refuses to consent to the sale and the others take action the costs will come from that persons 1/4 share.

"I've a feeling the widow will make an allowance for adult 4 who lives with her, as all the others have their own homes, adult 4 looks after the widows daily needs."

This is kinda key point. Adult 4 has not just lived in the family home but has cared for their widowed mother on behalf of all siblings. It would be very odd if that commitment of time & work was not recognised in the will. The idea of leaving the house to all 4 to split evenly would not recognise the value of that commitment on it's own. The mother would hopefully be leaving all her other assets to Adult 4, which would be sufficient to either buy out the others or when added to the split, sufficient to get suitable accommodation elsewhere. If they are not, the will will be perverse.

That would go to a moral duty but there are a lot of factors at play. Adult 4 has been living rent free for years as well. Some parents believe that equality is equity.

I wouldn't. Watching out for and eventually caring for someone elderly is a very significant financial contribution. If you want to crassly monetise it, look up the costs of this sort of care provided by commercial carers. The rates per month far exceed rents many times over.

In fact traditionally, this was exactly the arrangement many entered into. One family took in the elderly parents and other elderly relations, minded them till death and then benefited from the legacy.

 'you want to crassly monetise it,'

I would suggest that is what you are doing, by expecting the widow to leave all her assets to that one child.

The person caring for their parent, is rewarded by living rent free for life.

Either way, the widow really should clarify what she wants to happen in the event of her death. And the adult in the family home, should be planning for their own future.