If your US citizen sibling (as long as he/she is over 21) petitions for you, you would fall under the Family Preference Category which has a waiting list:
Congress has limited the number of relatives who may immigrate under these categories each year so there is usually a waiting period before an immigrant visa number becomes available.
The amount of time you would have to wait depends on a number of possible factors:
- The demand for and supply of immigrant visa numbers
- The per country visa limitations
- The number of visas allocated for your particular preference category
If one of your parents petitions for you, you would fall under Green Card for a Family Member of a Permanent Resident which is a lower preference category than being petitioned by a US citizen (a lower preference category means it will probably take even longer).