I'm in the process of inviting my sister during my delivery, I applied and took a interview date, I was thinking to include a invitation letter, baby due date confirmation letter from my hospital, my bank statements, relationship documents and a letter from my sister's current employer for her employment verification in nepal, but in confirmation slip it clearly said "letters of invitation, land titles, property valuations, police reports, audits, and documents from
financial institutions) are strongly discouraged", Now I'm thinking what documents could be included? hospital due date confirmation? relationship document? or any other document? Sister's employment letter?

No documents needed. There will be 50/50 chances of getting Visa. For sisters, chances of getting visa is slim.

Do not invite your sister to help with your delivery. That is a job that can be available to a lawful American resident so the visa officer at US embassy in Nepal will right away reject such request. However, invite your sister to explore the USA. After she arrived here, she can help you with delivery. Please send your tax return document from last three years and bank statements. No other documents are needed.

I guess they will issue visa for your sister if she has decent travel experience in (australia,europe etc) and high paying job/business in nepal which tie her to return back otherwise jaya sambho!