Brenda, what is the cultural background of your husband and Mi? Do they come from a culture where it is expected that on widowhood MiL will automatically move in with sons/DiLs? Do you share the same cultural background? If my supposition is right she is just repeating a pattern she has grown up and assumes she will continue.
Otherwise, unless she is disabled, and I do not get the feeling she is, I cannot see why a woman of only 75 (my own age) would want to just pack up her own home and live as an appendage to her son & DiL's household.
Obviously life immediate after losing yor husband is traumatic, but now he has been dead a year, could she not consider moving into a small flat near you and livng independently?