As someone who has experienced sunburns, "You'll fry if you don't be careful" does make sense, but only if you said it in the right context. For example: A: It's so sunny today! I'm going to the beach. B: Make sure to put on sunscreen. You'll fry if you don't be careful. "You'll fry if you're not careful." might be a little more natural though.

What do you mean? Sunlight is bright but is not necessarily hot or frying. An example of such is sunlight on a ski slope. Do you wish to say sun damage on your skin? Or dehydration? Or just feeling hot? What kind if "care" do you wish the reader to take? It is always best to write simply and clearly. For example, "It is hot today. Wear appropriately cool clothes." Or, "The ultra violet factor is high. Put on your sunglasses and your sunblock for protection." One is not sure what you want to say with just an indication of "sunlight...fry...be careful". It is very vague. Think about what you really want to say. Say it simply. That is the best English.

It is interesting to note that Pelin's is a half-baked copy from www.oxforddictionaries.com. Here is the full (and correct) original: "1.2 [NO OBJECT] informal (Of a person) burn or overheat: Example: with the sea and sun and wind you’ll fry if you don’t take care" This is a classic case of a student not helping herself by not giving complete information. If she had said, "I saw this definition and example from [where]...and I am wondering whether I could vary it like so ... and still be correct", then everyone would immediately have had a common context to work with.

A more natural way to say it is: "You'll fry if you're not careful" or "Take care of yourself, or you'll fry!"

This was quite a good question, I thought. The answer is, "You'll fry if you don't take care." English people don't say, "You'll fry if you don't be careful." In fact I have never heard, "don't be careful." in a sentence that I can think of. Yet I have heard "don't be" followed by an adjective, eg. "Don't be so hard on yourself." or "Don't be stupid." So you don't always need a verb after "you don't" as in "you don't take care". Odd, isn't it? It's something to do with the word "careful", I suppose. If you really wanted to use "careful" you could say, "You'll fry if you're not careful."