In the country where I live, indeed where a parent has a child(ren) and there is a split and there is maintenance involved, when subsequent children are born, the first set of children's maintenance is NEVER reduced. If a parent decides to have more children, the subsequent children must be maintained entirely on whatever is left in income AFTER the first set of children receive their maintenance. This is how it is in many countries in Europe.
50/50 is not always possible, but if a parent moves away (there are few one night stands) then the parent who moves away will usually work out a way of keeping up as close to 50% with that child they've chosen to move away from. They've be well out of the country at 400 miles though :-) Usually if this happens, the parent who gets less time has to compensate further the parent who has the greater time.
The entire maintenance system here is set up not on a stupid 15/20/25% of income as basic maintenance from the non resident parent or less resident parent, but the "cost" of the child(ren) is ulated and divided according to parental income. Housing costs are divided amongst the number in household, school costs, food costs, all sorts, it is all added up. For example, each of my children costs me 500 euro per month. The father never sees the children so all costs are paid by me. The court decided the father must pay 60% of their costs and that is what he pays every month. If his income dips, too bad, he still has to pay!!!!! If he gives up his job, too bad still, the court still makes him pay. Absent parents are almost non existent here.
The OP's BIL sounds a really good father, despite the distance. He has taken responsibility and the sister unfortunately has a lot to learn.