U.S. citizens can petition for parents (once the citizen is 21), children of any age, and brothers and sisters (once the citizen is 21). Green card holders can petition for a spouse and unmarried children, but not for parents or siblings.
Some relatives are "immediate relatives" with no yearly limit: spouses, unmarried children under 21, and parents of adult citizens. Everyone else waits in a "preference category" with a yearly cap. The State Department's monthly Visa Bulletin shows where each line stands, by category and country.
That is why your sister's case can take far longer than your mother's. The wait in some categories runs for years, and for some countries more than twenty. We file it right, protect the priority date, and tell you what to do (and what not to do) while the line moves.