I worry abut my parents but they don't care?

My parents are already at high risk because of their cholesterol, and my stupid sibling purposely buys deep fried, artery clogging junk food for them and its annoying me. I care about my parents, and I tell them for their own good that they should eat healthier. But they just ignore me and laugh about it, saying 'they don't eat it all the time'. Even so, when they're not eating McDonald's every week, they replace it by eating full fat cheese or cookie rolls at night. They're not obese, but they're going to get there if they continue. Help me? I'm starting to give up, but at the same time, I don't want to.