Legal Question in Wills and Trusts in California

If your mother added your brothers as joint tenants, she cannot just take them off. If she dies, her share of the property passes to her joint tenants. If they are willing to deed or quitclaim back to her, she can title her share of the property in a living trust. She can make you a co-trustee, or successor trustee, of the trust, but not a joint tenant (joint tenancy is a way of holding title to property which generally includes right of survivorship).

Re: your dad's share of the house- if they hold title as tenants in common, he has testamentary power over his half, in other words, your mother doesn't have a "right" to it. But if he dies without a will, she would be the intestate heir if it is indeed community property. If it is separate property (one question is whether the joint tenancy severance resulted in each of your parents holding a 1/2 share as separate property) it will pass (with no will) 1/3 to your mother and 2/3 to you and your siblings.