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I'm sorry to hear about your situation. To be entirely honest, as it stands now, you don't have any rights. That is, a power of attorney over financial or health matters is presumptively valid, unless someone challenging that POA can establish that the person giving that power did not have legal capacity to know what he/she was doing. Now it's possible that you can go to court and get a guardianship established over your father, showing that his competency was impaired due to dementia when he signed the POA, and thus be able to rescind transactions that they entered into on his behalf. But until then, the POA (and thus the transactions) will be considered to be presumptively valid.
You can go to your county's family law facilitator to get assistance with filing for guardianship: http://www.courts.ca.gov/selfhelp-facilitators.htm
Again, you don't have any rights merely by being his son or a sibling to be involved in these transactions. The POA leaving you off would be the presumptive legal relationship, and that would only be able to be changed by a court ordered guardianship.
Hope that clears things up a bit. If you have any other questions, please let me know. If not, and you have not yet, please rate my answer AND press the "submit" button, if applicable. Please note that I don't get any credit for the time and effort that I spent on this answer unless and until you rate it positively (3 or more stars). Thank you, ***** ***** luck to you!