Legal Question in Wills and Trusts in California

I wouldn't like to make a definitive pronouncement without seeing the paperwork, but most likely you are correct. If the house was left to your sister outright and free of trust, then she had the right to put her husband on as joint tenant, and it would pass to him by right of survivorship on your sister's death (regardless of whether she had a will.) BUT if your mother's will left the house to your sister IN TRUST or as a LIFE ESTATE, with you and your brother named as remainder beneficiaries after your sister's death, you might be able to contest the joint tenancy transfer.

What is important is the language of what property interest was passed on to her sister. if the Will states it goes to her unless she was deceased before your mother, then she got a fee simple interest and can do what she wants with it. If she got merely a life estate [to sister for life and then to my two sons] then upon her death the life estate ends and the property goes to your brother and yourself. So you need an attorney to look at the Will.

My prior answer addressed this new aspect.

The property belongs to the husband by right of survivorship. He will need to record a document called an "affidavit of the death of a joint tenant" together with a death certificate, to make title clear in his name.

Thank you for providing more information.