US Citizen - sponsoring green cards for sibling and family

[texascitizen]

Hello - My wife and I recently became citizens and my wife would like to sponsor green cards for her sister and family (sister, husband, 2 kids ages 11 and 6) who reside in India. I have been going through USCIS website and found information regarding family based green cards. I came across category for siblings but it does not mention anything about the siblings children. Assuming she can sponsor for that family of four, would the kids be ineligible to get green cards, if they do not get approved before they turn 21?

Also, how much would be approximate costs for the whole family? Please advise and please point me in the right direction (any links would be appreciated).

thank you!