She was absolutely right to marry Mr C. God he's dull, but he was unlikely to beat her, torment her, lock her in an asylum, abuse her children, or flaunt his mistresses. By marrying him she gets status, the brand of independence that being married brought in those days, and she will never have to watch her children freeze or starve to death.
When you read first hand accounts of life back then (or a little later, in the Victorian era- Mayhew for example) there's dozens of stories of decayed gentlewomen who are essentially starving in attics because, somewhere along the line, things haven't gone well for them and that's where they end up. If you've been bred to run a household of staff, be an ornament in the drawing room, and produce children, you're good for nobody if you don't do just that. These women probably couldn't boil an egg, they were meant to be helpless and dependant.
Charlotte, if unmarried, probably would have been OK in that she'd go to live with a brother and be free babysitting - a depressing life, but she probably wouldn't have starved or lost caste. The Bennett sisters on the other hand, would have been absolutely fucked if they'd got to old maid status. No brother to take care of them. No independent fortune. The elder girls would have married shopkeepers or impoverished vicars if they were lucky, and the youngest would have ended up selling themselves to the regiment. Mrs Bennett was a shrewd woman in her way- if she lived now, she'd be hothousing those girls with extra tuition for scholarships to private schools and mandarin classes on a Saturday.