Given the circumstances that she could be in her low 20's and studying, it might be difficult to get granted a visa, but should try will all possible documents n hope for the best.
Following should be required at a minimum:
1. Your letter of invitation and affidavit of support for your sister's travel and temporary stay in US
2. You bank statements, income verification, tax returns, empliyment verification
3. Your proof of legal stay (h1)


4. Your sisters legal status proof in Germany
5. Letter from your sisters college saying she is studying full time and that period in September is College break and that she is enrolled in the semester following the break
6. You and your sister's relationship proof- family relationship certificate probably from Nepal ( how does the consular know u two are siblings??)
7. Who is supporting her financially in Germany? U or ur parents, affidavit of that and proof of source of finance to prove that she will return back to germany to complete her study