1) yes, absolutely, this will be my fourth or maybe fifth will to date1) Wills are temporary. You can update your will as circumstances change.
2) Avoid messy situations. A house owned by three people is messy. If that means excluding your eldest from the house, so be it. It sounds as if she does not need it.
In particular, it would be messy if two children own it jointly with their sister's widower.
3) What happens if you die tomorrow? Presumably the other parent will look after the two minor children?
4) I presume that there is some mechanism - a trust - whereby you can leave the house in trust to someone who might not be able to manage their finances themselves - so that the trustee looks after them and makes sure that they don't sell the house and buy Bitcoin with it.
2) want to avoid that. Current will has each child equally receiving assets, but needs to be updated now as situations are changing
3) this is a good question. Ex is their guardian, however lives in different county and they dont want to move their. He could potentially move in with them to minimize any disruption
4) yes I think thats whats needed. I think its just stated in previous will that assets are held in trust, but id like to understand more what this means.