If she drinks diet drinks, then you could appeal to her intelligence and ask her why, for the love of God, does she consume aspartame. I doubt if she will listen to you at all about Vit D. Maybe she'll read about it somewhere as it's in the news constantly.

Other than that, no. I hate to break this to you, Andromeda, but you can be abrasive and his whole family seems to have chosen foods styles to cling to, regardless of what you might say. They'll treat you like a Jehova's Witness and not hear a word you say.

You don't bear responsibility for her or their choices.