Careful: the last sound is not a schwa.
"Brethren" is an archaic plural for brother. Nowadays it's only used in religious contexts. If you would use "brethren" in the English context -- "brethren in Christ," for instance, or talking about monks, then yes, it would be correct. (And "Brüder" is in fact the correct German translation for "brethren.")
Chicken is simply a diminutive of chuck or chook, both of which are still used in UK English though mostly by older people and rural types.
When I was kid eggs were commonly referred to by children and by adults speaking to children as "chucky eggs". I had assumed this was just kiddie speak until much later when I found out about "chuck" and "chook", and I realised that it was probably more to do originally with differentiating hens' eggs from others such as duck eggs.
In some colloquial forms of UK English, children are addressed as "chicken" by older women, eg "Come here, chicken". A few years ago a vet nurse even addressed one of our rabbits as "chicken" while she was taking her spay stitches out ("Hold still, chicken".)