If the US citizen has already applied for I-130, got approved and priority date is current, the sibling in Nepal will get the information from US embassy in ktm to prepare for visa application which will take only few months and it is not called i485. If the sibling is already in US, he will apply for i485 which will again take few months.
But, if the US citizen is thinking of sponsoring his sibling now, he has to apply for i130. i130 processing time is around 5 months but the current wait time is 12 years for US citizen's sibling before he can apply for visa for green card or before they can apply for i485 if already in US.
For instance, if the US citizen apples for sibling's sponsorship (i-130) today, the sibling can apply for green card in 2027 per current time frame.