Is it bad that I don't want my sister to have a kid?
Depends on your reasons, why don't you want her to have a kid?
She's too young to take on such a large responsibility.
Even though it might be a mistake, you cannot tell your sister she can't have a kid. She will be upset and stop you from seeing her kid after it is born.
I'm going to take that risk.
Before you say something that could change your relationship, take a few moments to plan what you're going to say. Rather than telling her what to do, calmly give her reasons that you are worried about her situation.