The men are reported to have blown kisses and passed notes before subjecting them to lewd comments.
The sisters, Aarti and Pooja, were then reportedly thrown off the vehicle as it was still moving and further assaulted.
No one came to their aid throughout the incident and a fellow passenger apparently said: "Leave these boys or they will rape you or pour acid. They will kill you and no one even get your bodies."
Part of the incident was filmed by another woman on board the bus.
One of them said: "We boarded the bus from Rohtak bus stand, this boy started teasing us by trying to give his phone number to us. When we protested, he called his friends and they started making obscene gestures. When I walked up to him asking not to make such gestures, he grabbed me by the neck while others held back my sister", reports Zee News.
"My sister then took out her belt and started hitting them. They left me and attacked my sister, then I went to save her. In the end they threw us both out of the bus.
"Passengers in the bus stopped us from calling the police. We were thrown out of moving bus & then conductor told us to file police complaint, but by that time they had all escaped."
Police have said the matter is under investigation but have drawn criticism for their slow response.
The girl's father, Rajesh Kumar, said officials were even trying to pressurise them not to pursue the matter.
India faces a huge problem with violence against women with numerous cases of gang rapes making the headlines.
Earlier this year two girls were found raped and hanged from a mango tree.
Some have even taken to vigilante justice as in the case last month when a man allegedly caught raping a young girl was castrated by an angry mob.
The situation is not helped by politicians that describe rape as "sometimes right".Suggest a correction