Bella, sounds like your sister in law is probably just 'one of those people'. Hopefully you can find ways to deal with her, and by all means, vent away here.
However, it sounds like you've not been in the country too long (or are you even here yet, I can't tell from your signature), so I'd caution you against generalising that it's a "British thing" (be it 'attitudes' or 'drinking culture' or 'rudeness'). The world is full of all kinds of people, from all walks of life.
It is definitely OK to ask if something is cultural if you don't know, but if you think about the nice traits your hubby has (would you consider him rude and cold for example?), try to remember that most of the population is generally full of nice traits
. It helps ease the transition and help to meet new people.