"Charles is related to you the same way I am to your son."
"And you are related to me in the same way Charles is to you."
Let's make it easier.
In the first sentence, let "you" = Bill, let "I" = Fred, and call Bill's son Paul:
1. "Charles is related to Bill the same way Fred is to Bill's son Paul."
Then, in the second sentence, "I" also = Bill and "you" = Fred:
2. "And Fred is related to Bill in the same way Charles is to Fred."
From 1, we know that
3. Charles and Fred are different generations.
From 2 + 3, we then know that
4. Fred and Bill are different generations.
If Charles and Bill are of the same generation, then Fred and Paul are of the same generation. But 2 makes that impossible. So
5. Charles and Bill are of different generations.
By 2, we know that Charles and Fred must be either both younger or both older than Bill. But 1 makes the former impossible, because of 3. Therefore
6. Charles and Fred are both older than Bill.
By 1, we then know that Charles is older than Fred.
The sequence in terms of generations is therefore:
- Charles
- Fred (1st man)
- Bill (2nd man)
- Paul
At which point we can reapply 1 and 2, and see that only steps of one generation will work.
Avuncular relationships are excluded, because then, by 1, Charles must be the great-uncle of Bill, and Fred of Paul; but that's incompatible with 2. Therefore the speakers are father and son, and Charles is the second man's grandfather.
MrP