This is a complicated issue and depends on several things: 1) Whether you and your siblings have any type of agreement (written or otherwise) concerning payments and ownership; 2) How title is held; and, 3) Whether there is a pre-marital agreement or marital agreement between your sister and her husband. Generally, property acquired prior to a marriage is separate property. However, if your sister has helped pay for property taxes, insurance, or anything related to the house, with proceeds from her job (i.e., community property), her husband may have some interest in the home if they were to divorce. Feel free to give me a call for a free initial consultation. 415-835-6777.
Disclaimer: You should discuss your situation with a local attorney, as the above statement is a general overview and does not take into account all of the factual considerations in your case. Nothing in this statement creates an attorney-client relationship.