I'm a mother of 2.. Elder son is 3yr+ n my 2nd girl is 7mth+ old..
I notice his rebellious n throwing tantrum is when my girl join in the family. At that moment, I was helpless n impatients as I need to look aft my baby girl who is still under confinement period. At first, I was thinking that he will end this nonsense soon. But I was wrong. He didn't and I know that I won't be able to control him or stop him from doing it. I seek help from my friend as she is studying children's psychology. She explain to me why my son is behaving that way.
Being a toddler or preschooler (especially the first borned), they are more self-centered. They don't really like to share their things with others. Especially parent's love. So by throwing tantrums or having a rebellious behaviour, they are actually trying to get the adults to pay more attention on them. I can understand for those who are working. They can't really have much time for their kids. But try to pick out some time for them. Kids are not like adults. Adult needs material pleasure but kids don't need them. We can get them the most expensive goods but to them, the most "expensive goods" is our love n attention. Try to spend some time with them. 30min to 1hr is enough. I mean is doesn't need to be everyday but twice or thrice a week. And this 30min to 1hr only belong to the 3 of u. (is possible). Don't allow any adult matters to interfere or interupt the time that u all are spending together. And keep telling them 'I love u' coz they juz want an assurance fror the adults.