What are the effects of eating just junk during puberty?

I need advice to convince my 16 year old sister to change her diet. I need an explanation on how bad her diet will do her in the long run and the consequences because I feel like she doesn’t believe anything will happen to her she keeps saying everything on the internet about diseases is fake because she’s been doing it for years and nothing happened to her and she also says she eats cucumbers so she’ll be fine so I want to prove her wrong that something would eventually happen.

She’s always been a picky eater. Her diet is basically two packets of spicy noodles with hot sauce for breakfast along with giant bag of hot potato chips and coke/Mountain Dew or energy drink to go with the meal for every breakfast. Sometimes she’ll eat Nutella for breakfast. Lunch is either KFC/McDonald’s or anything fatty and full of oil or pasta with hot chili sauce and cream. For dinner she’ll have the same two packets of noodles and a soft drink. Her snack is hot Cheetos or blue takis or literally just a lemon cut in half with salt and she keeps adding salt and licking it off the lemon slice, and the only healthy thing she’d eat is cucumber.

She does have a family history of diabetes. I know not everyone is going to get it but does her diet even provide adequate nutrients? How will her body react to this diet in the future? Is there any evidence of someone with the same diet who suffered long term effects?

What are the effects of eating just junk during puberty?