I don't think full brothers could have such variation, and the fact that my younger brother's package is a definite upgrade plays into the theory that maybe mom was shopping around for a better deal.
There are several ways to approach your central question: Did your mother give your father the shaft?
First of all, you seem to be of the opinion that sexual characteristics are inherited only from the parent of the same sex.
The facilities were spartan and we all ended up in a communal shower.
I'm sure this was the first time all four of us were naked together, and it was certainly the first time I'd seen my younger brother naked since he was little.
My stepfather is slightly younger and significantly healthier than my mother, and if she dies before he does, he and my half sister—their child—are her sole beneficiaries.