It is very hard to give a generalized answer to a specific question. You are better served talking to the lawyer who handled your relative's case. Each case is very fact specific and there must have been more pros to accepting the plea than the cons, to include a much longer sentence. Many folks charged with these crimes are otherwise law-abiding, upstanding citizens, but the law does not permit the conduct. That is obvious and reasonable. The second chance your relative will get it a shorter sentence and an ability to return to a law-abiding life. Hopefully, reoffending will not be an issue.
The facts that you have presented suggest that your family member, pursuant to 42 U.S.C. § 16911(2), is classified as a "Tier I" sex offender, and with good behavior, pursuant to 42 U.S.C. § 16915(b)(2)(A), his obligation to register as a sex offender for a period of fifteen years may be reduced to ten years. - Joshua Sabert Lowther, Esq., National Federal Defense Group.
With the exception of a pardon, you question regrettably comes to late. After thoughts are not unusual but are frustrating to convicted defendants and practicing attorneys alike. The time to be aware of and consider the ramifications of a plea and conviction is before not after the judge accepts the change of plea. I realize that my answer is not helpful to your family members situation.
This answer is rather meant to warn the readers of this forum that there is no substitute for early attorney evolvement.
Of course, every answer or response is based on the information provided in the question asked and requires a much more complete context than is available in this public forum. This answer/response should NOT be relied upon to make any legal decisions. Seek the advice of an experienced Federal and/or state criminal defense attorney in your jurisdiction BEFORE you say or do anything.