Visa for parents and sister (working for techM)

[darshanpandya]

Hi Folks,
I am on h1b with my wife on h4.
I would like to apply for tourist visas for my parents and sister. I am sponsoring them for their visit.
Visit will be for 3 weeks.
My parents run a business with mother as proprietor and father as employee in the firm, and my sister works as a management employee for TechM.

I have a couple of questions.
- Should I apply for all of their visas together or have my parents and sister apply for visas separately ?
- Since I am the sponsor would the tax, bank and other financials of my sister and parents be required and presented ?
- If applying together, Should I apply for their visas in Hindi (mom dad will be more comfortable) or english (sister will be more comfortable) ?

Please let me know what other precautions can I take to avail them the visa w/o an issue.

Thanks for your inputs in advance.