What Constitutes as "immediate Family"?

Due to the Coronavirus, my fiance and I have had to make the difficult decision to postpone our wedding celebration and reception. However, we're still getting married in an intimate ceremony on the date we've been planning for since March 2019. We want it to be small to keep things safe, but the problem is that my fiance comes from a HUGE Irish-Catholic family with lots of aunts, uncles and cousins, and we're very close with all of them. What is considered immediate family, and how do we explain this to the people who won't be there to watch us get married?