I want to tell my sister not to eat tofu, it's nasty
Why are you gonna prevent her to eat tofu? Do you have a philosophy against it?
Yes, because she looks good with a healthy body
Even though I think that your sister can eat whatever she likes, I support your decision to tell her about not recommending eating tofu. It's good that you care for her and wanted her to eat more healthily, but I don't think that you can stop her if she really wanted to eat tofu. You can just be there for her and probably monitor her diet moving forward.
I just want to tell her not to eat it.
As long as you aren't forcing her to not eat tofu, looking out for her health is really sweet of you. Eating too much of one thing is bad, and I hope she can realize that from you.