Do all the siblings need to be involved in health care decisions for their elderly mother in California?

No, as Jessie stated so well one or two is more than enough. To many people leads to confusion and often makes decision making much harder. Do as Jessie recommended and get all her paperwork in order thats very important.

Too many chiefs would end up causing confusion. Your mother should choose one healthcare proxy, with one more as backup if something should happen to the first. Your mother or her proxy will be responsible for healthcare decisions. The proxy should be one that would be in agreement with what your mother wants.

If your mother is still competent and a proxy has not been chosen, have her do her advance directives and set up a healthcare proxy (healthcare POA). You may want to check, also, that she has her financial POA chosen. Both of the POAs are very important.

No.