Anne has an advantage over Charlotte: her mother's friend, Lady Russel. Lady Russel would happily take Anne in and, most likely, provide for her in her will. Lady Russel probably has the right to will her own marriage portion and she has no children.
This is true. Unless Lady Russel dies first, and her property passes to a male nephew rather than to Anne.
It just goes to show how precarious all of their positions are.
Even Anne's friend, the widow (Mrs. Smith?) did everything right -- went to private school, married a rich upper-class man, lived in high society. Except her husband squandered all his money and died, and now she lives in ill health and poverty.
Everything was so tenuous in those days, and the sad thing was how few options people had to change their circumstances. They couldn't just go get a 2nd job, move and buy land elsewhere, or any of the normal options today.
In most Regency marriage contracts, the women received a settlement. This was money that the husband held for his wife, with several restrictions on how it could be used. However, if he died, it reverted to the wife, not the heirs. In Pride and Prejudice, Mrs Bennet has a five thousand settlement, which would go to her daughters, not Mr Collin, when she died. It was specified in Lydia's marriage contract that she would receive her share of this (her mother could not disinherit her - as though Mrs Bennet would cut off her darling married daughter!).
So Lady Russel most likely has money of her own, from her marriage contract. Her husband's estate was most likely already inherited by his heirs when he died and she lives quite genteelly on her marriage portion. Since she doesn't have children, I imagine the main estate was inherited by one of those pesky cousins that show up on such time, who would be less inclined to give financial support to Lady Russel than her own children might.
This is also the reason why elopement was such a bad idea - on marriage, anything a woman owned became the property of her husband and he had control over it. A settlement could ensure that her 'portion', or assets which she had previously owned, could be held in trust for her and her children / heirs, which would give her some protection in the event that her husband turned out to be bad with money, or if he died. If you eloped, or married clandestinely, there was no settlement and this left the woman, and her children, potentially very vulnerable.
Lydia Bennet was very fortunate that between Mr Darcy and Mr Gardiner, a settlement was made before she actually married Mr Wickham - in all probablity, it would have meant that the share of the capital settled her would be held in trust, and she (and therefore Mr Wickham) would only be able to get hold of the income, and unable to touch the capital, so she would never be completely destitute.