Legal Question in Real Estate Law in California

If you and your sister are the only beneficiaries as to the property in the Trust, you could probably just decline to take the 10% interest. It is possible that stil might leave some confusion as to what happens to the 10% so you can file a quit claim deed [should be able to find a copy on line or go to stationary store] in her favor. That should be filed with or before the title is changed so it does not have to be changed another time.

Inheritances can be disclaimed within certain time frames and in certain ways. This is all covered in the Probate Code. In particular, you should look up Probate Code sections 275 and 279. I think 279's subdivision (c) will apply to you, but I do not practice in the field of wills, trusts and estates. It is possible that the state-law disclaimer time periods are longer than those allowed by the IRS to escape tax consequences of ownership and its subsequent relinquishment.

Section 280 of the Probate Code covers how disclaimers are filed.

If you don't disclaim timely and properly, you will be deemed to have acquired the interest, at which point you'd need to transfer it by deed, probably a quitclaim deed.

Quitclaiming may be simpler, but a disclaimer keeps you out of the loop entirely, avoiding any tax consequences if done seasonably.