Sadly Karine as no children can be found for John and his wife his niece and nephew seem to be his only relatives if they will not answer you there is nothing we can do about that.
On RootsChat we look for grandparents, great grandparents and so on back as far as possible, we are not in a position to deal with living relatives.
It may be that the niece and nephew did not receive your letters, I'm not sure the people on the USA board can help any further but perhaps if you add to the earlier thread that you have not received a reply to you letters someone may be able to find something but data protection makes finding living people very difficult which of course is for the safety of all of us.
If the niece and nephew do not want to know about their cousin in France that is their right no matter how disappointing.
