In RWBY Volume 3, Chapter 4: Lessons Learned | Rooster Teeth - YouTubeWinter hits again the first time was in Watch you see Winter asked Weiss "How have you been?" Weiss was telling her, and Winter for no reason hits her.
This question is difficult, because regional and cultural differences will tend to impact what one perceives as "Abuse."
However, the situation does incline itself to being interpreted as abuse simply because of the position of power that Winter occupies within their dynamic: because Winter is established as the definite authority figure as the older sister, when she hits Weiss, it has an uncomfortable resemblance, for the audience, to abuse.
Whether it is abusive is up for debate, though it probably isn't as abuse is derived around asserting Power while Winter is trying to teach Weiss... although hitting her is still wrong.
This is similar to when Killy "revives" Cibo's machine-corpse by bashing her head against a wall in the Netflix adaptation of Blame! in the fact that it looks like abuse because Killy holds the (literal) power in that relationship (though, to be fair, it was worse in Blame! simply because of Killy's apparent complete and total apathy towards Cibo's fate in that film.)
It was wrong for Winter to hit Weiss, yes, but somehow obviously she's not abusive really. I'm not sure how to prove it other than to point out it's clear Weiss loves Winter, she hates their father who was confirmed to be abusive, and Weiss would live with Winter than stay with her father. Proven by the fact that in volume 5, Weiss was trying to go to Winter after she left her father who practically kidnapped her at the end of Volume 3. But you can see how their father was abusive in this video.
Parent or not, he had no right to hit her, much less hit her scar, on top of the fact he hit her harder than Winter did. Really, Winter treats Weiss better and Weiss loves and looks up to Winter (Which I don't see as the acts of someone being abused) especially since in comparison, Weiss hates her father who did abuse her as stated multiple times in the show. (If only I could find the clip where Weiss said this.) then I prove it was stated.
No. She is very authoritive but she's not abusive. Winter loves Weiss but sometimes Weiss can be disrespectful. Snapping "I Can't!" When Winter was trying to teach her something so Winter had a right to slap her.
No. If Winter was abusive much like their father was:
Then Weiss would likely be angry at Winter same as she is constantly angry with their father. But Weiss doesn't act like she feels Winter abuses her; she loves Winter and rushed to her when Winter first showed up and also Weiss didn't want Winter to go back to atlas it seemed because she was upset when Winter told her she was leaving. Now no this doesn't make it right for her to just hit Weiss but maybe that was her acting out of anger simply because Weiss being disrespectful. However, it's not Winter's fault entirely it was the way they were raised which means it was mostly the parents fault, and that also means that likely Winter was abused by their father as well.