I agree with you on this, but i am not so certain that this would be labeled as jealousy?
I do think it would be called naive, if one failed to recognize this, if someone is truely encroaching on ones family unit. I don't see this as jealousy, more like protectionism, and this is not a feeling one might have towards their partner, but the radar is up, for the person encroaching and you may give a heads up to your partner that they need to raise their radar too!
i hope that made sense! hahahaha! if it didn't, please permit me a ''second'' shot at it!!!!
jd