When Grandma Shaw makes a move, it inevitably ends in death or injury; at worst, she sends Luna far away, never to return to the Imperial City.
But who is Richard Shaw? Now, the entire Shaw family relies on him for support. He is already one of the most powerful people in the Imperial City; how could he always be subject to control by others?
He's already compromised once over the marriage. To Richard Shaw, even if Luna isn't his "white moonlight," there's still the fact that her brother saved his life.
If Grandma were to act when Luna is facing online defamation, it would turn into her framing Luna and Grandma dealing with Luna. Doing this would only completely infuriate Richard Shaw.
At that time, Grandma would be seen as a controlling elder, she would be a scheming wife, and Luna would be a pure and untainted beauty amidst chaos.
