Legal Question in Real Estate Law in California

You can force sale of the house through a court action known as a partition action.

As far as you renting out the room, she would need to approve of who lives there, since she is living there, too. On the other hand, she cannot prevent you from living there.

In a partition action, the court will direct sale of the house, usually through listing with a broker rather than courthouse-steps type auction. The liens will be paid and the net proceeds distributed to the former owners based upon a formula that takes into account relative ownership percentages and reimbursement to any former co-owner that has borne a disproportionate amount of necessary expenses such as mortgage payments, taxes, insurance and necessary repairs.

The problem with doing partitions these days is that so many houses are "under water" or have very little equity that there is nothing to distribute after selling commissions, liens and legal expenses are paid.

The problem with renting out your room is that all co-owners have an undivided right to shared possession of the entire property, including each room. Of course, while you were sharing possession on friendlier terms, you had an understanding about which room was hers and which was yours, and indeed co-owners can modify their rights of possession by contract. Unfortunately, unless your dividing-up-possession agreement is in writing and covers this situation, you can only rent what you have - a right of shared possession - and as a practical matter, that ain't gonna work unless the other co-owner is in agreement.