There is a house involved that was completely in my mother's name.
How do you know?
Have you obtained copies of her deed and any subsequent deeds from the county recorder of land office?
If not, then you don't "know" anything.
If it turns out that the house remained in her name after she was married then you have the option of opening probate under intestacy and asking the court to appoint you representative of the estate so you can probate her separate property.
You, her surviving spouse, and your siblings will all share in the inheritance of her separate property. Spouse gets half and you and any siblings share the rest.
If there is a will and it is more beneficial to the spouse then you action will compel the spouse to file the will with the court and probate her estate properly.
If it turns out that she changed the ownership to put her spouse on the deed as joint owner with right of survivorship then, yes, he owns it and can sell it to you or to complete strangers if he wants to.
So, the answer to "What should we do?" is first look up the deeds and get copies so you know where you stand.