How can I get around a controlling, mother-blocking sibling in order to update Durable Power of Atty for someone with "dementia."

I am very confused. You say that your Mom is "all there". How can that be if she has given DPOA to your sister, who you say has dementia? As to dual DPOA it is almost always a mistake as it means fighting, pure and simple. Does your Mother have DPOA for your sister? Or did your mother GIVE your sister DPOA to act for her.
Could you please clarify this. With your Mom and your sister living together, and your MOM in her 90s the only one who is "with it", and you living in another state this is going to be a mess any way I can see it moving forward. You are, I am afraid, going to have to make a visit, review documents, and see a lawyer with your Mom and your sister, I am afraid. If your Sister and your Mom will not share information with you and your mother is indeed in control of her faculties there may be nothing you can do at present. And if both of them are NOT in control of their faculties you will have a real mess on your hands as POA in another state. Wishing you luck

The way I read the question controlling sister won't share info with the OP and avers that mom has dementia therefore can not add to or alter the current document.

Your mom can create a new PoA at any time, as long as she has her right mind. YOU can't make a DPoA for anyone. Is your concern that your sister is going to take advantage of her financially? Or is neglecting her? I think it's good to pursue this so that there is transparency in your mom's life affairs. But maybe your sister knows something that you don't, which may be that she DOES have some decline and you can't tell since you don't live with her every day. This is very common. But your sister should be willing to make all transactions on your mom's behalf as transparent as possible.

Depending on how future phone calls with her go, you will need to make a trip out there to visit. I wouldn't announce that you're coming so that she can't "prepare" or make excuses why you can't visit. Make sure your visit includes a few week days so that if you are able to get an appointment with an attorney (do this in advance) or to get a new PoA signed, you will have that option. Your sister doesn't seem cooperative and the more you insist the more she is going to resist (probably). But if you are diplomatic, calm, and keep discussions unemotional, non-accusatory and focus on it being about helping her care for your mom and just being an informed participant in her care decisions, you may get more cooperation from her. Maybe.

If you show up in person and she won't let you into the house or talk to your mom, I'm not sure if you can call the police. You will need to check on your options. Also bring the PoA papers with you if you don't see an attorney. If your mom is illiterate but has 100% of her mind, I would opt to have the paperwork drawn up with an attorney. If you're not willing to spend some money to get this done, then just let your sister have her way and keep in contact with both of them. FYI your mom should be paying for both the plane ticket and attorney, if she has the financial means. Also, I've read elsewhere on this site that if siblings have too much power-struggling over a parent the state might decide to step in get guardianship over your mom and then neither of you will have any say over anything in your mom's life. Wishing you a good outcome for your mom.

I don't know the set up in CA but in CT a POA turns into DPOA when the criteria is met. That criteria most always requires at least one doctor saying the patient is no longer capable of making decisions for themselves and that tends to go along with not being able to be left alone, some require more than on MD sign off on the "durable" portion go into affect and or an attorney either period or when there is not a meeting of the minds on it's need among family. My mom gave all three of us POA and MPOA, though my brothers asked that I be the primary decision maker on medical stuff (yea I get the tough decisions) but she has directives too so not totally up to us from my perspective but my point being yes a person can have more than one however if there is not a good relationship as it sounds here I wouldn't recommend that. To create all of this official paperwork your sister and your mom should have seen an attorney who drew this all up and would have determined for themselves if this was what mom really wanted and she was able to decide that, remember to kick in the durable part at least one doctor has to be willing to put it in writing that time has come too so you have at least 2 outside professionals to consult on what they heard and saw from mom and sister. I would ask who the attorney is that did the paperwork and if indeed it was done that office should have a copy if your sister doesn't.

I will say that it makes the most sense to me that the sibling who is doing the day to day care, especially if she is living with mom is usually the logical and realistic choice for POA for all kinds of reasons and if your sister is taking good care of mom I wouldn't rock that boat. Still if she isn't willing to share info and open about what is going on with mom you have every right to be concerned and that is going to require a trip to CA for a visit where you can see mom, spend time with her and see just how she is doing. You can meet with the doctors and attorney as well just to make sure you are clear on everything, for instance I haven't been asked for POA papers in 3-4 years until recently again and I couldn't find them anywhere. I know I have them somewhere "safe" but I just broke down and emailed the attorney who drew them up and asked for a copy, it's on their computer.

Maybe offer your sister a respite for a few days and let her know how much you appreciate what she does and how hard it can be, if she refuses that and discourages you from having face to face contact with mom then I would do what someone else mentioned and not give her details about your plans, she will be on notice you want to come for a visit so she can't say it's a total surprise and you could make hotel arrangements so she isn't having to host you then but call from around the corner and grace them with your presence. If you can find a meeting place with sister great, if not and she is still being cagey just go on to the professionals not on an attack trip but on a fact finding one because sister is so overwhelmed...and then you can go from there, you may find your sister is doing everything you would for mom and does have it handled she simply is overwhelmed and not good about the business of detail clarity. You can help her be better about that and I hope that's what you find but even more so I hope you have a good visit with mom!