NTA but you need to calmly explain your reasoning to Jacob
From what you write, I'm assuming Jacob used to be an only child, and has been for almost 18 years. He's obviously having a hard time accepting the fact that he's not the only one now, that he has a sister and your attention is split. The fact is, and 18th birthday is a huge thing because it should represent adulthood. Jacob childish tantrums show that he's very much nowhere near it. He will resents you FOR SURE but there's also a possibility that he will realise once he'll reach young adulthood, how childish and unfair his attitude was, and your reasonings. Giving him a "small punishment" won't do anything, and he will probably feel like he once again got away with his tantrums.