The name Brocklebank and Brockbank are still here where I live and my knowledge of the geography helps ,
I am using a few different resources ,I am not 100% sure your man is from Lancaster,my thoughts being more likely Lancashire,
there is another marriage a few years earlier ,but I haven't found deaths for these ladies,
Matthew could be Johns nephew and grandson of Sarah ,I will post snippets and you can decide if they are relevant,
I must point out that any thing I do post must be checked and not taken as gospel,