I can't presume to speak for Bern in any way, of course, but I wonder if she thought that they might be reasonable and see her right, but once she realised that her brother/the others wanted to screw her over, she added up all of the costs that she'd incurred (including lost earnings) and saw it as her chance to get that repaid, or lose it forever.
The brother is quoted as saying "The only agreements that we (her siblings) were aware of was for my mother to give Bernadette £50 per week 'keep' and to ensure that Bernadette was not to be out of pocket." - so maybe this was actually her just taking him at his word and reimburse herself for all of her very substantial costs.
Also, when you house and care for somebody, it involves a huge amount of costs that can't be easily isolated and receipts provided for. How many times do you hear of deadbeat dads who complain that "She just spends the money I give her paying HER bills, and not on the kids" - expecting her to use it to buy 'nice things' for the children, rather than actually providing them with a warm home and food in their bellies?
The insulting £50 a week reminds me of a post in a CF thread once, where somebody had stayed with the poster for weeks and had full board, meals, been driven around and more... and then, when they were due to leave, they mentioned that they'd bought a pizza for a snack, but wouldn't have time to eat it before they went home; and it cost them £3 and was in their freezer, so they actually expected their host to pay them £3 for it, whether they wanted it or not!