"These are your grandfather and grandmother."
Translation:ये आपके दादा और दादी हैं।
अपने is a 'reflexive' pronoun, used to refer back to ('reflect') the sentence's grammatical subject; in this case that's ये, so using अपने would be like trying to say 'they're their own grandparents'.
(But it doesn't necessarily have even that sort of maybe-usable for comic effect meaning in Hindi, it might just be gibberish, I'm not sure.)
When you separate the words with an और, it's a toss-up between the two but in the hyphenated form to say 'grandparents' (because Hindi does not have a single word for this), दादा-दादी is more common. Similarly, नाना-नानी for maternal grandparents. However, while saying 'parents', the mom comes first - माँ-बाप or माता-पिता.