My problem with Sera's reaction to Cole being accepted into the party is less, "OMG MY BABBU LEAVE HIM ALONE YOU MONSTER", and more, "oh, honey, if you think that's bad, you better buckle in". I am a fan of Cole's, but that's not really the apprehension I have here with Sera.
I play the game in a pretty chaotic way. I like to push what people think is true about the world to see what happens. I like to take the skeptics approach on in-world beliefs and use everything as a learning opportunity, even if that comes with a risk. It seems like that is basically the opposite of what Sera wants, so I see us not seeing eye to eye a lot. It's not like I'm shocked she feels this way. Well, maybe a little surprised. I expect a lot of characters to react poorly to my play style, but I guessed Sera would be less up tight about it. That's more so just my expectation for her to fit the "rogue girl" stereotype, more like Isabela. It's not as if I don't think her reaction fits within the context, but it does make me wonder if we'll be able to get along. I'm not really against it being so, really. In fact, it's nice when a character doesn't fit expectation, in some ways.
I would be happy if Sera became more flexible over time, though. I know her interview says she's fairly untested in her beliefs and inexperienced. I think it'll be interesting seeing where she goes with that.