I'm wondering if my sister and her boyfriend are still together or not
Why don't you ask them?
Honestly I don't want them to be together anymore
You may have a good reason for that, but it's her relationship and if no harm is being done then I don't see the issue.
I just don't like they are together anymore, they are going to hurt each other.
Why don't you just talk to your sister about what you feel. It's better than wishing ill about their relationship.