Legal Question in Landlord & Tenant Law in California

If your sister is so disabled as to be unable to request a copy of her rental agreement, the first thing you need to do is get a guardian/conservator appointed to take over her legal and financial affairs in general, because this issue will come up again and again in dealing with disability, social security, banks, etc. So someone needs the legal authority to take care of these things for her. The guardian can be you, another family member, or anyone who has a genuine interest in taking care of these things, is capable of doing it, and will put her interests ahead of their own in dealing with her legal and financial affairs. If she has no money and little income (eg just social security disability) the county may appoint a public guardian, though that is not going to get the best attention to her needs.

Once there is a person legally able to act on her behalf, the rental agent is probably right that she cannot break the lease without any impact. However, if the guardian does break the lease for her, the landlord is legally obligated to re-rent the property as soon as is possible using commercially reasonable means, at the same or higher rent if possible. She is only liable for the time from notice that she is abandoning the lease (and emptying her things out of the apartment) until it is re-rented. That should be a relatively modest amount, maybe less than her deposit. If it is more, and I were in your shoes, I'd say "go ahead and take a poor disabled woman who is under a conservatorship to small claims court and we'll see what they say." But technically, legally, she would be obligated for that amount.

One last thought, you probably should get her things out of the apartment asap, and tell the rental agent in a letter that she is abandoning the apartment, even before there is a conservator. Even without the full legal authority to act on her behalf, the informal notice coupled with the physical abandonment of the tenancy will probably be treated by a court as putting the landlord on notice that he has to start re-renting the property as soon as he can.