I used the words "accept Christ" because I was speaking of Arminians who view their salvation from that (IMO erroneous) perspective.
The Calvinist says --
SALVATION IS OF THE LORD.
That being the case, human beings of "normal" intellect and abilities, like everyone born, are equal in the eyes of God -- fallen creatures, dead in sin, who are ONLY redeemed through Christ's sacrifice by the gracious gift of faith WHICH IS FROM GOD ALONE.
We become aware of our salvation when we realize our heart has been turned from stone towards the desire to live a life that reflects God's glory. It is all and only of Him.
Thus, the salvation of your unfortunate cousin, just like that of the bushman and the imbecile and the three-day-old infant, is one of those perplexities that's beyond human understanding, and is EASILY and CONFIDENTLY left to God's own counsel.
However, I'm much more concerned about the pass you seem to give others who knowingly blaspheme the holy and sovereign Trinitarian God of all creation.
That's an easy one.
Boy that really sounds like a BURNING BUSOM confirmation to me, Doc.