Hi,
I like to preface long replies with a short answer. But in this case no short answer suffices, so pardon my long ramble.
Jealousy is complex. There are a lot of ways you can model it. I'll outline a few.
Let's say the two females are Ann and Barb, and the third character is Charlie. Ann flirts with Charlie, and Barb gets jealous.
One way to implement this is to define Ann's flirting as a deed with negative impact because it threatens Barb's potential relationship with Charlie. So the deed "Ann flirts with Charlie" is really "Ann threatens others' potential relationships with Charlie." Ann flirts with Charlie. Barb sees this. Since Barb likes the deed target (Charlie), she has a negative reaction to Ann. The catch here is that you don't want to report this deed to Charlie, since for him it might be a positive thing.
Alternatively, you could frame Ann's flirting as a betrayal of Barb's friendship. So the deed isn't "Ann flirts with Charlie", but "Ann betrays Barb," which has a negative impact.
Or you could define a second faction member for Charlie, say called "Charlie's Availability", that represents the up-for-grabs feeling that anyone could hook up with Charlie. Give everyone who's interested in Charlie a positive affinity to Charlie's Availability. Since flirting starts to solidify a relationship, it negatively impacts Charlie's Availability. When Ann flirts with Charlie, report the deed "Ann hurts Charlie's Availability" with a negative impact. Anyone who likes Charlie's Availability will lose affinity for Ann. In this case, untick the FactionManager's Can Witness Self checkbox so Ann won't judge herself badly for reducing Charlie's Availability.
The ideas above work within the existing framework. Another possibility is to add another relationship trait and change the deed evaluation function. You could call this trait Cooperativeness or, flipping it, Competitiveness. Ann and Barb, despite having high affinity to each other, would have negative Cooperativeness when it comes to romance. Barb doesn't want to cooperate with Ann flirting with Charlie. But maybe Ann's mother wants her to connect with Charlie, so the mother would have positive Cooperativeness. Your new deed evaluation function would have to factor in Cooperativeness when evaluating certain deeds like flirting.
You just had to ask about the trickiest emotion, huh?