Do you think I should tell my gf to tell her sister not to come over for weekends?
Is her sister causing problems? Is that it?
She gives me an impression that she is very negative and pessimistic.
There are some people in my family circle that I do not want to spend time with as well, so I feel like I understand where you're coming from. However, telling her to not come may hurt your girlfriend's sister's feelings, so I advise that your girlfriend tell her very politely and suggest that she visit next time.
No way, she needs to make sure that her sister doesn't come again.
How does your girlfriend feel about her sister coming on weekends? If your girlfriend doesn't mind her sister coming over on weekends, you really can't force her not to see her sister. Could you just make alternative plans for yourself when you know her sister will be coming?