Situation: My mom, a widow, owns a small ranch property equally with her sister. Mom's 50% interest in the property is held in a living (AB) trust. Her sister holds her 50% interest in her own name and it is not part of the community property of she and her husband. Mom's sister is not comfortable thinking about death etc. and has refused to set up a will or trust.

Question: If Mom's sister were to suddenly pass away, who inherits her 50% interest in the ranch? The property and the sisters are all in California

If Sis dies with a will, her property goes according to the rules of inheritance for CA. Youi can easily find them by goggling that. Generally, the estate is divided to a spouse and children.

Is Sis married, husband gets it
Does Sis have children they get it
If not I think it goes to her brothers and sisters

Situation: My mom, a widow, owns a small ranch property equally with her sister. Mom's 50% interest in the property is held in a living (AB) trust. Her sister holds her 50% interest in her own name and it is not part of the community property of she and her husband. Mom's sister is not comfortable thinking about death etc. and has refused to set up a will or trust.

Question: If Mom's sister were to suddenly pass away, who inherits her 50% interest in the ranch? The property and the sisters are all in California

A lot of it too is going to come down as to how the deed is titled. Do you have any idea what the wording on the deed says ? You might also explain to your moms sister that by using a living trust she can avoid probate and also her taxes will be lower or possibly none at all. Because the property is held separate then the title becomes most important as it may be that if she should pass it would pass to your mom and not to her husband / children.

Calif recognizes separate property within a marriage.