Onceler headcanon…? Idk just listen me.
So, firstly I think that Oncie is older than Brett and Chet.
I think that twins mom really loved twins dad but the twins dad died after few months they born. Then she meet with Onceler’s dad, which is rich and have a child who is Onceler. Onceler’s dad’s money is only thing she wanted, that’s why she married with him. Then she poisoned him to just get his money.
What she didn't expect was that Onceler’s father bequeathed a really large portion of his money to Onceler and demanded that even one person involved in this money go to prison until Onceler turned 18.
And that’s why she always hate Onceler but try to be nice just for the money.
And when Onceler became 18, he never give that money to any of them, instead he immediately goes to find a place to make thneeds when he graduates.