The big issue in your situation would be that both the brother(s) and sister(s) have one blue shell gene and one white. That means any offspring hatched from the breeding have a 25% chance of not getting a blue shell gene; which means they'd lay brown eggs (or possibly white but that's not very probable). That's assuming the original sire is pure for blue. You could also breed brother with mother and then breed those offspring back to the original sire. I say just go for it and see what you get. It's fun to experiment.
Before I start I want to clarify vocabulary. Americauna is not a breed, It's a term used by hatcheries to trick people into thinking they are buying something they are not. They are Easter Eggers (EE). I could write a whole bunch about EEs but the big thing to remember with them is that you can't really know their egg genetics just by looking at them. People then buy and breed "Americaunas" and perpetuate the lie, some intentionally and some not. Let me state very clearly that I love EEs, they are great birds, but they aren't a breed. Ameraucanas (notice the spelling) on the other hand are an excepted breed that has defined traits and come in only a few specific colors. There are some hatcheries that sell true AMs.
There's no way the Chochin crosses were sired by an AM because they both lay brown eggs. An AM carries two blue egg genes, which means every single offspring will receive a blue egg gene. Blue is dominate over white. Not sure how much you know about egg color but a brown egg is a brown coating over a white shell and a green egg is a brown coating over a blue shell. White and blue eggs have no colored coatings. There are differing shades but that's the basics. They may have been sired by an EE but he would had to carry one one blue gene.
Based on your description of the pair, they both sound like EEs. Their colors don't sound like any excepted colors. There's a chance they were breed from different colored AMs but I'm guessing the breeder just had EEs. Given that, there's no real way to know what kind of egg genes they carry.
If your rooster has one blue gene, his offspring have a 50% chance of receiving that trait. Any females that receive it will lay a green or blue egg, If he has two blue genes all of his offspring will receive it and all female offspring will lay green or blue. Of course things are also dependent on the genetics of the hen. If she lays a green or blue egg she must have at least one blue gene. If the hen lays brown it will only pass white shell genes on to her offspring. So basically, you won't really know until the offspring start laying.