I feel certain Louisa is the eldest, and in my mind Caroline is the youngest, but others here have made good arguments that it could be Charles who is the youngest (or they might be twins).
Depending on the plot line I'm pursuing, I sometimes change canon to make Hurst a good guy (and a preexisting friendship with Darcy is the source of the connection to the family) and his marriage with Louisa is also good. In this case his canon description is just an incorrect impression people form - he eats too much and pretends to get drunk and sleep around Caroline because he can't stand her, and his 'deception' avoids arguments. (One joke that I came up with in an unfinished story is that he is the only man in London who finds Miss Bingley intoxicating.)
I find an interesting way to think of plot variations is, instead of taking the descriptions the narrator gives as being truth, to take them as a commonly held view which is actually incorrect. For instance, instead of Bingley taking Netherfield by chance, suppose he met Jane Bennet somewhere and fell in love with her, took the estate to be close by, and didn't tell anyone (or just didn't tell Caroline) because he was uninterested in arguing, and some or all of the Netherfield party believe the narrator's version.