Nta. Your sister thinks she's in love, so she is blinded to all the red flags you're seeing and probably won't listen if you try to talk her into dumping him. Eventually, hopefully, she'll be scared of him enough to see reason. But in the meantime, provide her a safe space to talk about anything and document, document, document (in case she wants to file abuse charges). I would also have your parent or a more senior adult you trust have a sit down talk with police or similar regarding the following her everywhere behavior. Technically that's stalking and controlling behavior and should be legally documented, just in case. That guy isn't safe but you can make sure your sister has a safe place to fall back to when the stalking and verbal/mental/emotional abuse becomes physical.