My partner (F33) called me a moron (M31) for eating her brothers (M20) shrimp at dinner

She (F33) and I (M31) have been together for 11 years. Her brother (M20) is staying with us for the weekend.

We went out to dinner tonight.

He offered me a bite of his rigatoni pasta – and I mistakenly ate a piece of shrimp instead of picking up a piece of pasta.

I felt bad as soon as I realized and apologized to her brother. Def not cool to eat his shrimp as there arent that many of them relative to pasta. She reacted by saying “you’re such a moron.” He de-escalated and said it was okay and not a big deal.

My absent mindedness or carelessness definitely upsets her and she has resentment over it and reacts to it in this way sometimes – but she doesn’t tend to in front of other people. I never like when she calls me names like this but I think it hurt me more because of that. I have asked her not to call react like this before – she has also asked me to not be so absent minded.

How should I approach a conversation with her and how do I / we fix this dynamic longer term?

TLDR: My partner (F33) called me a moron (M31) for eating her brothers shrimp at dinner. I am too absent minded and she is not nice in how she reacts. How do we fix this?