Caroline Onassis is not a Kennedy by marriage? Well, gosh, who knew?! Thanks for letting us know.
In common usage the expression that someone is related “by marriage” connotes that someone is not a blood/genetic relation but is connected by the later fact of someone’s marriage......
Thus, it sounded like you were asserting that CK does not carry any “genes” from prior Kennedys (Joe, Sr. and Rose Kennedy, etc.) but was an Onassis first. In fact, she was a Kennedy first, and only an “Onassis” in a very limited sense by her mother’s later re-marriage (from I understand she NEVER considered Aristotle Onassis her step-father and had a very strained relationship with him).
I really don’t know what you are asserting, but maybe you can explain.