Karl was Marie's only natural child.
Karl died in 1975, Frederick wasn't adopted until 1980--as a grown man. CNN has stated that Frederick has said that he was a friend of Prince Karl and that the grieving mother adopted him out of kindness . It generally seems to be accepted, however, that it was purely a business transaction since the Princess was very hard up for money.
I doubt that any of Karl's children have anything to do with this person.