Don't tell me you've fallen for him?! I won't stand for it. I refuse to accept that man as my brother-in-law!
Literal
Surely-not fell-in-love [polite-んですか]. Not-accept, not-accept [emph-ぞ]. That man [obj-を] brother-in-law [become-になる] [like-なんて], I [topic-は] absolutely hate [polite-です]!
A theatrical outburst in mixed register. まさか 'surely not / don't tell me' introduces the unwelcome possibility as a question; the dropped-ん contraction 認めん (from 認めない) repeated with the assertive ぞ gives the blunt refusal its emotional weight. Note the loose grammar あの男を義弟になる — strictly it should be 義弟にする (transitive 'make into') or 義弟になる with が; the を here is a natural speech slip. なんて attached to a nominal clause expresses dismissive incredulity ('the very idea of...!').