Yup, sounds familiar! Your child is jealous and at his age incapable of perceiving reality (adults are bad at this, too, but kids are worse) through the filter of his immediate experience.
So you can distract, "The important thing is that you got to read a story! I love reading to you. What did you like best about Dr. Piggywiggle Meets a Monster?"
Or reframe the goodness of being second: "Some might say staying in the bath longer is good, because you get to enjoy it longer."
And reinforce: "Your mommy and daddy love you both so much! We love giving you snuggles after bathtime and we love reading you stories. For us, that's the most important thing. Do you want a snuggle right now, I know I'd like to get one from you!"
Sometimes at my worst I say, "OMG, stop it! That's nonsense and you know it. Sometimes you're out first, sometimes your sister is, but either way, it DOES. NOT. MATTER. So please calm down. I can't listen to silly crankiness right now." <-- This is me out of exasperation, but I don't think it works as well.
