Sajhababa: I have seen many students from Europe granted visitor visas to visit the US. I think it definitely is worth giving it a try. The key it to provide evidence/documentations to convince the consular that your sister will leave the US after her authorized stay here. Some of these documentations might be proof of enrollment from college, proof of tuition payment (and/or housing payment), any scholarships she might have, proof of employment, if any, letters from college/professors, her grades, evidence of financial support from Nepal etc. that will prove that she has enough ties in Germany and thus higher chances of returning back after her brief stay here.

Additionally, if asked or situation arises, she should be able to tell the consular that she has everything to gain by returning to Germany and nothing to gain by staying in the US.

Good Luck to you and your sister.