No, you are confusing residence with physical presence. She should go and take care of your sister.
Mr. Shusterman is a former INS Trial Attorney (1976-82) with over 35 years of immigration experience. His response to your question is general in nature, as not all the facts are known to him. You should retain an attorney experienced in immigration law to review all the facts in your case in order to receive advice specific to your case. Mr. Shusterman's statement above does not create an attorney/client relationship.
That requirement is directed at the situation of someone moving to another state/city within the US and applying there. For example, some people prefer to apply in seattle, because it is faster, but they aren't allowed to do that if they haven't been living here. If your mother doesn't break other residency and presence requirements, and intends to retiurn to Seattle, this one isn't a problem.
This reply is intended only as general information and does not constitute legal advice in any particular case. This reply does not create an attorney/client relationship.