Complex sentence: Drake is Ross Poldark's brother-in-law and this was noted heavily against him (almost like a crime committed) and has a working class background, therefore he was deemed an unsuitable marriage partner for Morwenna.
Split into simpler sentences: Drake is Ross Poldark's brother-in-law and this was noted heavily against him (almost like a crime committed). He also has a working class background, therefore he was deemed an unsuitable marriage partner for Morwenna.