I would politely point out that the work requirement takes priority over a social engagement.
The "you have to be there!" would be understandable if you were at home and had no other more pressing engagements, but as you are a fair distance away on a work related conference, your in-laws should be far more understanding - particularly if the shower had been arranged after the conference was booked.
Id never have expected my husband (to be) to show up at what is usually a girls only event!