Tell that to my RC relatives. If the Church was serious about that, the membership lists could be edited immediately and everyone not on the list treated as lost pagans. How many would still be in attendance after that? In the Chicago area I'm betting you'd lose more than half. When I see that happen, then I'll take your "automatic excommunication" seriously. Until then, I see it as a mere rationalization, a bit of paper that formalizes the fallacy that "no true Scotsman" would ever behave so badly.
Peace,
SR
If they were on FR I would gladly do so. since they're not (to the best of my knowledge) you can give them this reference. https://www.ewtn.com/expert/answers/abortio2.htm:
Exact wording from the CCL:
TITLE VI. DELICTS AGAINST HUMAN LIFE AND FREEDOM (Cann. 1397 - 1398) Can. 1397 A person who commits a homicide or who kidnaps, detains, mutilates, or gravely wounds a person by force or fraud is to be punished with the privations and prohibitions mentioned in ⇒ can. 1336 according to the gravity of the delict. Homicide against the persons mentioned in ⇒ can. 1370, however, is to be punished by the penalties established there. Can. 1398 A person who procures a completed abortion incurs a latae sententiae excommunication.
Your relatives are certainly free to ignore this, just as many people choose to ignore speed limits or laws about driving intoxicated. sooner or later it will catch up with them.
God always gets the final word and the last laugh.