Yes, Marcos can apply for J-1 visa and do not need to cancel Petition for Relative Alien. That is Family Fourth Preference (F4): Brothers and sisters of United States citizens, and their spouses and children, provided the U.S. citizens are at least 21 years of age.
According to USCIS now is Now Processing Cases with Receipt Notice Date of April 30, 2001 - https://egov.uscis.gov/cris/jsps/Processtimes.jsp?SeviceCenter=CSC
There will be concern that the fact of submitting the petition would provide his intention not to return after J-1 visa expired. He need to be very specific in preparing evidences that he WILL return back to home country.
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