They can apply for reentry permit. It's going to be quite costly as a reentry permit costs $445 pp. http://www.uscis.gov/sites/default/files/files/form/i-131.pdf

But reentry permit will only allow them to stay out of the country for up to 2 years. They can reapply for another reentry permit one more time but they can only apply for it in the US.

They do have an option of applying for fee waiver but to qualify, they'd need to be receiving some sort of govt. welfare (SSI, Food stamps etc).

Staying in Nepal continuously for 3-4 years even with reentry permit might be a risky move and might end up losing green card status if you don't have a good reason of staying in Nepal. It all depends on how the immigration officer thinks of them when they return back. The main point they wanna hear is the stay in Nepal was temporary and for a given reason. The fact that the father/husband returned the green card, went back to live in Nepal, maintained his job in Nepal and the wife/daughters also accompanied them for 3-4 years might trigger an alert for the immigration officer that the wife/daughters did not want to maintain US as primary residence.
I would suggest your sis and her daughters stay in US for at least a year even after your brother in law returns back and then go back to Nepal later for max of 2 years.

Maintaining Permanent Residence:
You may be found to have abandoned your residence status if you:
- Move to another country intending to live there permanently
- Remain outside of US for more than one year without obtaining a re-entry permit or returning resident visa. However in determining whether your status has been abandoned, any length of absence from the US may be considered, even if it is less than one year.
- Remain outside of US for more than two years after issuance of a re-entry permit without obtaining a returning resident visa. However in determining whether your status has been abandoned, any length of absence from the US may be considered, even if it is less than one year.
- Fail to file income tax returns while living outside of the US for any period
Declare yourself a "nonresident" on your tax return.