Nothing in your post indicates a need for a pre nup at all.
Err... this right here:
My nephew and his squeeze are talking marriage.
That's enough. Protect yourself from evil angry be-yotches.
“Nothing in your post indicates a need for a pre nup at all.”
It clearly said that he is about to marry someone. She can wake up one morning and ask for a divorce, for any reason, or no reason at all. He would then be on the hook for half of everything he has. There are fine reasons to remove the profit motive from that.