As a U.S. citizen, I can apply for P.R. for my in-laws, who are in their 70ies. Being over 65, once they get P.R., they pretty much can get welfare and all health benefits
Some info on legal immigration: Most legal immigration needs a sponsor. Since you are talking about in-laws, you can't sponsor them. A Citizen can sponsor parents, minor children, brothers & sisters. If your wife is a US Citizen, she can sponsor for her parents. Sponsoring requires executing a legally binding, "Affidavit of Support", wherein, the sponsor agrees to pay for all expenses of immigrant for 5 years. Now, if they are indigent after 5 years, they can probably apply for SSI/Medical-Medicaid etc., But it is definitely not "get off the boat/aircraft, rush to welfare office" scenario that is sometimes portrayed. I do know many illegals use food stamps, that is because they have minor dependent children, most likely born here in US.
