Someone is raising that question, and Ed Peters answers it:
Any talk of Jackie’s excommunication centers on her October 1968 wedding to Aristotle Onassis. At the time, Jackie was (tragically of course) a free-to-marry Roman Catholic. Ari was a baptized non-Catholic divorced from another woman and thus not free to marry (1917 CIC 1069). The Jackie-Ari wedding took place on a private island and, as near I can tell, disregarded canonical form (1917 CIC 1094). In two respects, then, Ari’s presumptively valid prior marriage and Jackie’s apparent failure to observe or seek dispensation from canonical form, the Jackie-Ari marriage was invalid.
But where might an excommunication have come in?
Three possibilities occur to me, though none of them concern what I imagine most people might guess, namely, Jackie’s knowingly entering into an invalid marriage. Assuming that action is itself a crime under canon law (a difficult but not impossible case to make) it is certainly not a crime punishable by excommunication.
He goes on to explain the possibilities. Read the rest.
Lest we forget, Mrs. Onassis received a full and very public Catholic funeral in 1994. Read more.