Could be the problem with my sister-in-law too.
For many years, she was a single mother who relied on the goodwill of relatives for childcare, etc whilst she was at work. Fast forward to when my brother met her. Very nice at first, then her outspoken attitude reared its head. I put this down to nervousness at being in our company. One of four daughters, her parents split when she was very young; two living with father, other two with mother.
Her behaviour has got worse since she and my brother started to brag about all the things they were doing, how much they cost, etc. Both have good jobs and company cars. Whilst it's a personal choice, I wouldn't have spent as much money on their house as they have but it was her decision to stay in the area she knew.
Apart from the drink problem, I considered she may feel uncomfortable in my company, which sparks the unacceptable comments/actions. My mother agreed it could be a reason, but it's still inexcusable. My aunt is constantly striving to be number one.
A family pecking order is dangerous for many reasons. If they want to behave in this way, then I'll gladly move over to not impede their route to the top!!