You did the right thing, and it would've been the right thing even if you weren't injured. Violence should always be a last resort, something you use only when talking doesn't work and walking away isn't possible.
Since you say they've done it before, has your sister tried to talk to the gym owner or even the police? If there are other regulars, especially women, who've seen the harassment happen often, I'm sure they'd back her up. If it's just verbal harassment, the police might not do anything, but if it's also physical, then they're likely to do something. If she can take a video of the harassment, without the criminals themselves noticing of course, that would help, too. And if possible, it might be good if she spoke to a female police officer. They're said to take sexual harassment more seriously than their male coworkers. There are always individual differences of course, but I don't think it would do any harm.
BeaArthur wrote:
Am I the only one here who thinks, she should go to a different gym?
Well no, you're not. That would be one solution and probably the safest option for his sister, but looking at the big picture, it would be great if those harassers got some consequences for their actions, like getting kicked out of the gym for good by the owner or get fined by the police... or get a restraining order, perhaps? While women (and men) who simply escape the situation when being harassed should never be blamed for it, it's always great when a victim stands up for herself (or himself) and manages to cause some harm (not necessarily physical) to the harassers, for that will discourage those people from doing it again and will work as an example to others who might consider harassing people.