You do not have to do this and do not have to make excuses. Even grandmothers are often not guided by logic or evidence, and beliefs are accepted, authoritative sources are also different. As it was in the series "Interns", a grandmother can only be convinced by a grandmother.
Nothing. It is impossible to convince grandmothers of anything, because they live with their convictions much longer than grandchildren. One can only explain to the grandmother that every person has the right to choose and gradually teach her to calmly accept someone else's choice.