Visit visa for USA green card holder with bangladeshi Passport

[rangers]

Hi Members
My parents live in America (on & off) for some time now. last year they got their USA green card. They are living with my sister. Sister do the financial andaccomodational support. They came here three times in the UK with visit visa. Lat time they came here back in 2020
i am planning to bring them here with visit visa
My query is now:
** as they have no tie with Bangladesh, do they still have to submit papers from Bangladesh?
** My sister and her husband will provide the financial documents/ sponsor from the USA. As parents has no income or even any bank account there? will it hamper their application
** I am civil servant here and my wife works in a school. We have three bed house with mortgage, what sort of papers do we have to produce from our side
** any other info or suggestion will be hugely appriciated.

TIA