Think the key word here is pattern. When something has been a set pattern for this long, it can take an actual action to break it.

If you guys are out together and a fight starts, just get up and leave. If she is at your house, make her leave.

Explain it beforehand as a team effort to improve things...that if either she or your wife start falling into the persistent habit of arguing with each other, this time together would end immediately. No ifs, ands, or buts....this time is over.

This did work for a relationship I was in, although the pattern had not lasted years long...more like a couple of weeks. We never did need to end an outing, but we did go into separate rooms a few time.

Unless she just refuses to try, and sincerely try, I wouldn't ban her from the birth of the baby.