I get where you're coming from @alpacachino and it doesn't matter if you're eating out at McDonalds, high end, or in between, it's the principle.
Posters are obfuscating. Your unwritten rule equates to most decent people's rule of thumb. You order in or around a similar value, OR, you chip in a bit more to cover the extra cost, particularly if it's quite a bit and personally, I think a tenner extra, per course, is a lot, regardless of how many split the final bill.
In my friendship group we all order exactly what we want and if it's out of kilter, we offer more. No one says please do, because so far as we're concerned, it works out knock for knock over time. We don't have anyone who plays advantage.
In your instance BIL is not only questionable as to how he treats your sister, which is colouring judgement of him, but he's a CF to be so out of step financially every single meal out and never offering to pay a fairer share.
If no one else is prepared to say or do anything, then do it for yourself. You say family will kick off, so they'll be unhappy, but the cost of that is that you're unhappy and possibly others in your group. I'd speak up because it's every single occasion, not just here and there.