An American citizen can sponsor their own parents or step parents for a US immigrant visa, but not their in-laws. LPRs cannot sponsor their parents (only spouses and children).
If the spouse moves to the US, they can be US citizens within 4 years, and can then sponsor their parents who would expect to wait a further year for their visa.
It is possible for people of independent means, including retired people, to live in the US on the visa waiver program. They can buy property, but can only stay in it for three months at a time.
There are plenty of details on the embassy website.