Actions of this nature are very much against the law; it was because of situations like this that our entire unlawful detainer (U/D or eviction) law came into being. No one came be put out of a legitimate tenancy with proper notice and subsequent court action. Turning the utilities off may also be actionable. Unfortunately, the facts presented are insufficient for me to give an opinion as to the ultimate result. If properly done, the eviction of your sister and neice may be successful, but only after due notice and court action. Seek out a knowledgeable U/D defense attorney.
Based on the facts you've given, the actions of your aunt are in violation of California law. The attorney you should hire should practice in the field of "Unlawful Detainers," also called "Landlord-Tenant." Hopefully, they will also have some experience with the probate code as well, since we're dealing with probate and conservatorship issues as well.
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