Someone will have to open probate and deal with his estate. There is nothing stopping either of your wife's sisters from starting that process themselves.
Note that the sisters won't necessarily get 1/3 of the value of the trailer, they'll get 1/3 of the value of the estate after all of the assets (the trailer, any bank and investment accounts, etc) are sold and all of the debts (funeral expenses, medical bills, the lien on the trailer if there was one, outstanding credit card debt, etc) are paid.
The life insurance isn't relevant as the beneficiaries receive it outside of probate.