Probate procedures in California are complicated. Your step father passed intestate, or without a will and a probate will need to be filed. Under the probate code, family relations of certain degrees must be notified. The children will need to be notified. If all of the assets are community property, then you mother will inherit as spouse. However, if there are any assets held in joint tenancy or separate property, your mother may not be entitled to those assets in their entirety. I recommend that a probate attorney be consulted in your area.
I disagree with Ms. Harper's answer.
Contrary to what Ms. Harper indicated, if the deed says the house was held as "joint tenancy", then it has already passed to your mother "by operation of law" and your stepfather's children have no claim on it. Your mother will need to record an "Affidavit of Death of Joint Tenant" in the County Recorder's office. Note that there might be some tax benefit in doing a "spousal property petition" instead of this.
So the first question is: how exactly was the house titled (you'll need to look at the most recently recorded grant deed [NOT "deed of trust"). Instead of a "grant deed", there might be an "Interspousal Transfer Deed" or a "Quitclaim Deed", although those are less common.
If the deed says the house is held as community property, then your stepfather's share of the house will go to your mother unless your stepfather had a will that left his share of the community property to someone else. Your mother will need to file a "spousal property petition" in order to make this happen. The filing fee for that is $350 and the lawyer's fees should run $1500 to $3000. Your stepfather's children will need to be notified of this petition.
If the deed says the house was held as "tenants in common" then your stepfather's share will need to be probated. If he had a will, his share will go to whomever he named in his will (but his children will still need to be notified and given an opportunity to "contest the will"). If there was no will, then the stepchildren are legally entitled to a portion of the house - they would receive 2/3 of your stepfather's share; your mother would receive the rest.
This information is not intended to substitute for professional legal advice and does not create an attorney-client relationship. You should accept legal advice only from a licensed legal professional with whom you have an attorney-client relationship.