You can sponsor your sister, but it will take many years. Once your mother obtains permanent residence, she can sponsor your sister if your sister is unmarried. The issue of your other nieces and nephews is more complex, and the legal custody arrangement you have described does not make them eligible for any immigration benefits. You should schedule a consultation with an immigration attorney to chart the best course of action for your family.
You will need to file a petition for him to immigrate to the U.S. There are several additional requirements that must be met for him to derive citizenship from you. I suggest you consult an immigration attorney before taking any action.
Never, ever, under any circumstances go to USCIS for legal advice! USCIS customer service agents are not attorneys and are not allowed to give legal advice. Contact an immigration attorney for a consultation about your options.
Did you not read the Affidavit of Support Form before you signed it? You would have had to list all immigrants you were agreeing to support. Schedule a consultation with an immigration attorney immediately to find out if there is any way to withdraw your Affidavit.
You won't be able to relocate to the U.S. with a visitor's visa, no matter what immigration status your children hold. Investment visas are complex and require a substantial investment of time and money. Schedule a consultation with an immigration attorney to determine if your business may qualify. Good luck!