I'm going to tell my sister that what she cooked tastes really bad.
Are you afraid that your sister's cooking is bad?
Yes, I'm not sure why but it just doesn't taste that good
There are a lot of times where I have eaten something that I did not really like, so I understand how you are feeling about your sister's cooking. But if you are going to be rude about how it tastes, I think you're just going to end up hurting her feelings. How about you be polite with her, and maybe try to cook for her in the future instead?
I'll just let her know that I don't like what she cooked.
The best thing to do is to tell her you don't like the ingredients, not necessarily her cooking. That way her feelings are spared.