1. Some anti-Covid-vaxxer people still claim the Covid vaccines were not tested, and it sounds like nephew buys into this. Covid vaccines were not "suddenly" authorized for use with adolescents and younger children. Testing with adolescents and younger children was done for many months before the vaccines were authorized for use with them. Quibble "long enough" as you like, just don't pretend the authorization was "suddenly" and untested.
2. However intelligent or unintelligent the nephew may be, if he was around his parents much immediately prior to their hospitalization he probably got Covid, just very mild (or he toughed out worse symptoms without telling anyone). IOW, he may have "immunity" from recovery, which is similar in effectiveness to "immunity" from vaccine. He could confirm this by doing an antibodies test, but it sounds like he would not be amenable to such a suggestion.
3. The general trend from Covid 1.0 and alpha-beta to Delta to Omicron seems to be one of decreasing deadliness (improving treatment knowledge and the effects of vaccines are part of that trend, making the trend complex in cause). If there has been a "good" time to get the virus and join the resistant herd, this could be it. It's not zero-risk, but nothing in life is, not even getting out of bed in the morning (the sudden BP change, the sudden heart rate change, stepping on Legos left in the hallway by the munchkins ...).