Can I apply to bring my parents, brother and sister to the US if I don't have a job now?

If you are 21 years or older US citizen, you can file petitions for your parents and your siblings. For your parents, they are considered as immediate relatives of a US citizen [you] and if they came to the US with a visa ( even if it has expired), they can adjust status [green card] in this country without having to go back. For them, you can file the I-130 petition and green card application at the same time. They will get interviewed for their green card within 8 months or less. Of course, you should be able to file I-864 Affidavit of support for them. If you are unable to do so, then, find a friend or relative (US citizen or green card holder) who is employed and can join you as co-sponsor for the affidavit. Filing petition for your siblings is under family 4th preference category. Visas in that category will not be current for at least 12 years or so. But the earlier you file it the earlier they can be in line. At this time, since their visas will not be current for the next several years, you do not have to file an I-864. All you file right now is just their I-130. Hopefully, when their visas become current, you will be working at that time and you can file the I-864 affidavit of support for them You should speak to an immigration attorney for further clarification on the necessary steps for your parents and your siblings respectively.

Unless you can show that you meet the financial requirements for an affidavit of support, you will need someone to agree to be a joint sponsor for your family.

You can petition for your parents and your sister. The processing times for your parents will be about 9-12 months; for your sister will be about 12 years. In both cases, if your income is insufficient, you will need to get a joint sponsor whose income/assets are sufficient to sign the affidavit of support on behalf of your parents and your sister when the time for visa issuance comes. (9-12 months from filing for your parents; 12 years from filing for your sister).

Yes, you will need a co-sponsor.