And the context is one in which you have people called the brother of the Lord, brothers and sisters of the Lord who hang around with Mary, and Jesus called "firstborn."
And yes, Thayer does say that it gets the force of a conjunction in his lexicon when used with ou.
Yet most times a name is given, the mother turns out someone other than Mary. That is because the word "brother" and "sister" is used expansively, either as spiritual brothers or as kinsfolk. Jesus called "firstborn."
As a legal term. He is fuirstborn, regardless of siblings.
the force of a conjunction
whatever the grammatical category, it does not speak to the issue of marital relations after Christ was born.
Your reading of the Scripture is possible. It is however, not the only one possible, and yours happens to be heretical. Deal with it.