Jealousness

We can never go on one day without any problem. Now Peter's new one, according to his teacher: jealousness of a new baby. Because the teacher was holding the new comer, Peter got very upset. At home, sometimes, he is jealous of Yaya being close to or sitting on me. Lei's comments"if there's someone who should be jealous, that should be Yaya.." He came to this world with Yaya's exsitence, how come Peter is the one?