Need some advice for how to bring this up again?

For background my SIL is worried about exposing my nephew to covid and asked me, my MIL and S/O not to go to his cousins for dessert Christmas Day because of it.

Me and my S/O decided to spend Christmas Day with my mom. She decided after speaking to my aunt that we’d go up there because she felt it’d be nice to see other people. I haven’t officially said yes because of my SIL asking us to not see their cousins. So I assumed seeing mine was out of the question.

I tell my S/O about going to my aunts and he got defensive and was like “I thought we were going to your moms?!” I explained we were invited and she wanted to see if we were invited.

I have to let my aunt know yes or no. How do I bring it up since he was so defensive about the change the first time? I’m trying to get better at communication and don’t know how to bring it up.