Your Dbro is on almost exactly what my Ex-P is. My ex-P pays £55 a week for 2 dc. Which is what the CSA say he should be paying. Granted, he also chips in for some of the school uniform, half of DS2's shoes (his shoes cost £50+ a pair due to his muscle problems), half of the cost of large, essential baby equipment for DS3 (having to buy it all again, large gap 7+ years between DS2 and DS3, DS3 very unexpected!). But only when given a decent amount of advance notice, and if he agrees it is essential, not something I want to get for DS3 IYSWIM.
So, he paid half for DS3's highchair, but I paid for all the cost of the secondhand jumparoo I bought.
There is NO WAY the CSA will make him pay the mortgage AND maintenance - your Dbro needs to get his name taken off the mortgage - she is trying to SHAFT him. Badly. He needs legal advice NOW. Because anything he pays on the mortgage will NOT be classed as child support - and if his ex goes to the CSA tomorrow, he will be liable for maintenance from that day EVEN IF HE CAN PROVE HE HAS BEEN PAYING THE MORTGAGE.
On what he is earning, he should only be paying out £55 a week for maintenance. Paying the mortgage is not his responsibility - if she wants to keep the house, she will have to get a FT job, won't she, or go into rented accommodation and sell the house.
Hang on - your Dbro has them overnight for 2 nights EVERY WEEK? Then it won't even be £55 he's liable for. 2 nights a week = 104 nights a year = 2/7 reduction in CSA calculated maintenance. . He should only be paying £39.29 a week.
I know far too much about how maintenance works...