Is there any legal way a schizophrenic patient can be forced to take a diagnostic medical test?

My sister is 58 years old and has been diagnosed paranoid schizoprenic since the age of 13.  For the past 7-8 years she has had symptoms of imaginary voices and a strong fantasy life.  She is somewhat medication noncompliant, although she does take a monthly injection.  She lives in a personal care home in Houston, TX.  She is on medicare and medicaid.  She was recently in the hospital for fluid on her lungs.  During her hospital stay, a large tumor was discovered on her breast as well as some suspicious spots on her lungs.  She agreed to an ultrasound, but refused a needle biopsy.  The doctors told me that they were very certain that it is cancer.  Without her signature, there was nothing they can do.  They also felt that she would not be candidate for surgery or chemotherapy because of her state of mind, although there might be some medicines that she could take if they could get a correct diagnosis of the type of cancer.  Basically, they just washed their hands of the situation saying if she didn't cooperate, then "nature would take it's course."  I do not have medical power of attorney over her. I am her payee for social security and SSI.  In fact, I am her only living family member.  I can not consult with her psychiatrist because she will not sign a paper of consent to consultation for me.  In fact, she will not sign any papers right now, because she is afraid she will have to do the biopsy.  She can not accept any bad news and will not face reality.  For example, both of her parents are now dead, but she will not believe it.  I really would like for her to do this biopsy and get an accurate diagnosis of her condition.  We have tried to convince her, but I do not believe she will ever agree to sign.  Her "voices"  have told her not to do it. She was released from the hospital and is back in the personal care home.  If her condition worsens, I am sure she will not be able to stay there.  They do not have a nurse on staff.  She can be a very difficult person to deal with if things do not go her way.  Is there any way I can help her legally get the tests she needs so that we can see what can be done about her care?  Are there any agencies out there who will give advice to family members?