The Ninth Circuit has determined that the Adam Walsh Act, which prohibits certain US citizens who have been convicted of crimes against children from sponsoring their non-citizen relatives for immigration status, can be applied retroactively to petitions filed before the effective date of the Act, if the petitions were still pending when the Act came into effect.
The full text of Gebhardt v. Nielsen can be found here:
http://cdn.ca9.uscourts.gov/datastore/opinions/2018/01/09/15-56072.pdf