i would inform her after she has been moved, and settled in for a month or so. perhaps post her a little note that is advising of your mums new address incase she needs to visit or send any correspondence. I wouldn't ring her if you can possibly avoid it, because it makes you responsible, and you will be the scape goat. Just send a generic, unpersonalised typed note advising of the new address, the same as you might send to some of her old friends, acquaintances, and don't put your name on it.
"Mrs Joe Bloggs wishes to advise of a change of address. She can now be contacted at the following address: xxx Phone: xxx"
I don't think anyone deserves to be given any information if they don't attempt to keep in contact and find out what's going on, regarding the well being of a person for the past 20 years, then they have no right to any input in decisions being made.
If she wants to complain, then she can complain to the mangement of the care home and the appropriate medical professionals and social workers, and she can come up with a suitable alternative care plan on her own.
