I need some help with this situation.?

Ok, my sister had her baby taken away by CPS about 6 months ago. Our mom called them on her because she is such a neglectful mother. My sister and her boyfriend, the babys father, didnt take care of their daughter and did some pretty nasty things to her while she was in their custody. Anyway, my sister, I guess can say who can and cannot see the baby. Well, the only person who cant see the baby is my mom. Recently my sister changed her mind and said my mom could see her whenever she wanted. Over the weekend however, my mom went to go visit her granddaughter and didnt realize my sister was visiting. My sister put on a show for everyone and yelled at my mom to get out of there, so my mom left. My mom didnt realize my sister was visiting. It was a simple mistake. Now my sister is complaining and saying she has taken away my moms visiting rights again. But I told my mom she cant do that because if she wants my mom to have nothing to do with her granddaughter she needs to have a judge say she cant see her and my sister needs to have a reason why she has ordered such a request. But, what I want to know is, doesnt my mom have like grandparents rights or something? Can my sister actually take away my moms visiting rights just like that? I mean, my sister doesnt even have her own rights...idk. But I still think it's wrong. If anyone has any info, please let me know. Thanks.