In this context, a family of a late grandfather is outraged because the deceased man decided to bequeath all his inheritance to a person which wasn't even a part of the family.
This conversation is between a person(executor of the will I guess)who was reading the will to the family
They are obviously angry and protest against it and ask if there are any safeguards against his decision and they question the sanity of the old man(they basically try everything to annul that decision). The sentence i've trouble with is from the person reading the will.
"You, not liking what he did does not speak to testamentary capacity"
What does speak to mean in this context?
Does it have anything to do with this def from TFD? 3. formal to give evidence of or comments on (a subject): who will speak to this item?.