I'm glad to hear that Prudence and Raven are getting along well. Here's some info that I found on the internet about female cats going into heat:

Feline Puberty Arrives:
The age a cat reaches puberty will vary from kitten to kitten, just as with humans; and much depends on factors such as environment and breed. Outdoor or feral kittens may reach maturity earlier, especially if intact male cats are around. Siamese cats can show signs of puberty as soon as 4 months of age, while other breeds may not reach sexual maturity until they're 10 months old. In general, by 5 to 6 months of age, cats have reached adolescence. Female kittens can then go into heat and become pregnant even though they're little more than kittens themselves. Cats being bred intentionally shouldn't be permitted to get pregnant until they're at least a year old, so they can develop the instincts and emotions to cope with motherhood.

If I were you I'd be making an appointment to have her spayed asap especially since you said that she's learned how to escape outside. Both of my girls were spayed very early and everything went well. They just need to have some time to heal. Good luck.