What to do about disrespectful sister?
Stay far away from anybody who is disrespectful.
just try to stay away fronn her as nnuch as possible
Sounds like you have some jealousy going on of your sister who may be taking advantage your mom. However, grand parents almost cannot help taking care of & loving their GKs. Now, to you. Dude, you never ever let your sister or anyone purely disrespect you. If you do, THAT'S ON YOU!!
People who are feeling confident and secure and have self respect don't feel "disrespect" others. She probably does not respect herself much. Maybe you can help her with this, then she might be a much nicer person.
Leave her get on with her life and you get on with yours. Some People get on with their siblings, some don't.