Edyth wanted to reward for Richard and Frances for taking care of her out of love and affection, and (let us suppose for illustrative purposes; the court does not make this clear) she wanted to give them the inheritance as an additional motive to take care of her.
There continued taking care of her in response to the offer would give Edyth want she wanted: the bestowing of a reward and the knowledge that Richard and Frances had an additional motive to continue to take care of her.
True or False - Therefore, it would be reasonable in the circumstances to view the offer as inviting acceptance by performance; to view the offer as saying, "I'll promise to change my will provided you continue to take care of me; just continue to take care of me and we you can count on inheriting."