You don't really seem to give us many details, and I reckon it's because you don't know them and your sister is not yet able to divulge them. Was this man a co-worker of your mother's, or just some random schmoe? How did he approach her? Did he try to lure her out of the 'safe' area somehow, or did he just drag her, or what exactly happened--we can't really paint a good mental picture with the amount of detail you give us, and neither will the police. I am assuming, since you say you have no idea what his name is, that he was indeed a stranger.
What exactly did he try to do, and where? Indoors, outdoors? Were there many people present? How exactly did your sister get away--if you can get her to go to the police and report this...that would be good, if she remembers what he looked like, what he was wearing, anything peculiar about his face and body, like moles, tattoos, birthmarks, facial hair or scars or other identifying features? What exactly did he try to do? "Molesting" is such a vague description, unfortunately, so I don't know how you should approach this, if you report it--since he didn't succeed, I guess it would be attempted sexual assault. Does your mother know?
As for your sister, do not make her tell you anything until she's composed herself. It will be difficult for her, and you should try to be there for her as much as possible. Convince her into seeking closure--reporting the incident, even if the guy didn't succeed in molesting her, it's still important, since he violated her personal space and tried to molest/rape her. I recommend that you seek at least short-term counseling for her, so that she can sort out what happened, and recover. It seems to be a good option.
Unwanted sexual contact is always traumatizing, no matter how old the victim or the attacker--so therapy is really what you want to look at, even if he didn't succeed in doing what he intended to do, he violated her space and made her feel unsafe--and that's traumatizing in and of itself. Like I said, try to be there for your sister as much as you possibly can, and don't force her to tell you anything she doesn't want to tell you--until she's ready.