While I don't know your financial situation or what country they are from, life can be very expensive and difficult living as, basically, an undocumented illegal immigrant.
They will not be eligible for any free government welfare, food stamps, medicaid, auto insurance etc. They will not be eligible for health insurance through the ACA. They will be living here on a cash basis. Look up cash prices to see a dentist, doctor, emergency room bills - they aren't pretty. A single incident can wipe out whatever money they earn for your siblings, which I'm guessing isn't that high given they would be working under the table.
I know of two couples who were able to legally bring their parents over here on an IR5, and it led to the breakdown of both marriages BTW, but maybe it will work out for you. My suggestion - unless their lives are in grave danger in their home country (in which case they should apply for asylum), they should go back.