Harold Acton, in The Bourbons of Naples writes very briefly that due to her sister's treatment, Maria Carolina wanted to make a complete break from revolutionary France (i.e. cut off all diplomatic communication, trade etc.), but was prevented from doing this by Ferdinand. If in this instance Maria Carolina was unable to sway Ferdinand, it would not have been possible for her to commit sufficient funds from the royal treasury or her personal jewels (again difficult to turn into cash if her husband vetoed this) to effectively undermine the French government by procuring the escape of their prisoner - and I assume she would have needed a lot of money to carry out an escape (Fersen spent his whole fortune on his abortive escape plan, I believe, and he was very rich). Moreover, even if she had managed to get some money together, she would have had to try to obtain Marie Antoinette's escape not using any established Neopolitan network of agents in France (as this would have been forbidden by Ferdinand) but via persons she could trust to work independently in a foreign country (and there wouldn't have been many of them). Such personal agents would have needed more time probably than Marie Antoinette had, to learn about possibilities of bribing guards, setting up a more effective escape route, etc. - and this in the teeth of what must have been much more effective guarding than previously. I think it was a much greater job than just raising enough money - and Maria Carolina did not have the backing in this case of the head of state. If he wouldn't allow her to raid the treasury, or sell her jewels, or use official diplomatic (or unofficial government) channels to help her sister, she was pretty much stymied. Ferdinand, when push came to shove, was the ruler and his views prevailed.