what you haven't explained is what percentage of the joint assets this £170 settlement is? Because frankly, on a salary like his, I think £170k is pretty shite after however many years they've been together (which I assume is at least 10 or more). Me and DH's combined income is significantly less and if we got divorced tomorrow, our combined assets, split 50:50 would be less than £170k each, but not that much less (together 17 years)
But then you also think she can buy a house for that 170k so I assume you live somewhere a lot cheaper... so what the devil has happened tot all this money. If the lifestyle of the family was facilitated by this in disposable spending, it's not weird for her to want spousal support. As otherwise I guess he'll continue spending huge amonts of glorious holidays away etc, while she's living a much smaller life and her DC are wondering why there's such a huge disparity between mum's house and dad's house and activities at both, especially if as it's 50:50 he's not paying maintenance so how do clubs, clothes, activities get paid?
Personally, I'd suggest your brother offer her a much higher once off settlement figure and perhaps a small amont of child maintenance (even though he's doing 50:50) to facilitate a closer similarity between lifestyles between his house and hers going forward.
As the significantly higher earner in our relationship, I am so tired of men bleating about how they "earned the money why should I give it to her". It's such a bollocks. when I worked for an investment bank, I can assure you, I could not have got promoted and received the salary and bonuses I did if DH hadn't been a SAHD. Since i left banking, my income, while still much higher than DH's, has definitely been impacted by the fact that I don't WANT to be available for work 24/7, which I was when DC were small.