My girlfriend, who recently had to help one of her dogs cross the bridge, was telling me how the other dog was coping.
Out of the blue, Sierra will take the "brace for chase" pose and look over her shoulder... only to realize her sister wasn't there. She is also apparently dropping her sister's favorite balls on her empty bed.
Nobody, but nobody, can tell me that animals don't grieve; and they don't "get over it" overnight, either. It takes time.
Hugs to all of your family.