How do I stop being the messenger between my two adult siblings?

My brother sent me a screenshot and wrote, "Can you explain to her why this is ridiculous?" Ten minutes later, my sister called to ask whether I'd tell him she wasn't changing her mind. They were arguing about where to hold Dad's birthday lunch.

I used to pass messages because it got plans moving. I know how to word things so neither of them immediately takes offence. This time I'd spent my own lunch break explaining a restaurant choice I didn't especially care about. Then my brother said I was clearly taking her side.

Dad hasn't asked me to organise anything. He would enjoy either option. My siblings both say they're only asking because the other one listens to me. I like being trusted, but I don't like becoming responsible for how two other people understand each other.

Can I step back halfway through a plan I helped start? I don't want to announce a new rule and leave Dad with no birthday lunch, but I don't want another week of forwarding carefully softened messages.