There could be some jealousy involved of course, but the word I think you're looking for is ENVY.
Pip appears to be somewhat envious of your life (your great, long-standing relationship, sex life, business/career and family life). On some level, she wants this for herself and almost seems to be trying to live it vicariously, by inserting herself into your domestic situation and making herself indispensable to you and madmaxx.
This thought had struck me earlier in this thread... however, her choice of words (quoted above) make me think this notion isn't too far off base.
Belladonna, you may have some esteem issues of your own and, as Kevin suggested, may worry that Pip will "replace" you in madmaxx's bed and/or heart. At times, you may feel somewhat threatened by the possibilities, understandably, even if you otherwise feel your relationship is solid.
However, for the reasons I mentioned further up the page, I think it's much more likely that it's PIP who has issues with envy, esteem and (not) valuing herself highly enough as an individual worthy of love.
I would definitely watch the situation closely and consider if her words and actions align. I'm not suggesting Pip is on a fast track to "Single White Female" or "Fatal Attraction"-like bunny boiler behaviour,
but I would be wary of just how far you allow her to insinuate herself into your home and "love" life, considering you appear to be growing increasingly uncomfortable with her behaviour, which you describe as erratic, and many of the things she says to you in relation to madmaxx.
I mean, does she want madmaxx himself, or just someone LIKE him, for herself? Are you "just" FWB (who is she referring to here: you/her, her/madmaxx, or you/her/madmaxx?) and if so, why does she describe you as "sister wives" - a term which implies much more than a casual arrangement/shared sex.
Your situation reminds me somewhat of another thread that appeared on the forum a couple of months ago, started by txgirl, involving her boyfriend and a good female friend. If you have the time and the inclination you may wish to read this through to the end and see if you can spot some telltale similarities in the behaviour and motivations of the "best friend" - even though the situations are not duplicates of each other:
http://www.polyamory.com/forum/showthread.php?t=97611
Help WHO? Pip? Why do you think you need to put your own problems aside to help HER deal with her issues?
And HOW do you plan to go about doing that when, as you admit, you yourself have some insecurity and self esteem issues going on, which this situation may be exacerbating rather than aiding.
It is true that Pip appears to have rose-coloured glasses on when it comes to her feelings for madmaxx (and possibly the "excitement" of the sexual situation between you three).
To some extent this is normal for the NRE phase of any newish relationship.
However, it does sound like she is "idealising" both him (madmaxx) AND the relationship that exists between you two - and she wants a piece of that delicious, mouth-watering pie for herself.
As your "best friend" she watched your relationship from the outside and no doubt heard you talk in glowing terms of yours and madmaxx's emotional closeness and passionate sex life. You and madmaxx gradually brought her in closer and closer, until at some point she decided she'd like some of what you're having. And with your joint blessing, she got it.
So, now what? You say Pip doesn't know exactly what she wants... and doesn't sound like you do either. Her uncertainty and odd behaviour is making you doubt the wisdom of your choice. I would certainly move VERY slowly and carefully from here onwards...
I agree with Kevin that you NEED to spend some regular time - preferably scheduled - in which you three sit down and discuss where each person's head is at regarding the relationship. This isn't a conversation that just happens once - communication needs to be CLEAR and ongoing.
DON'T be afraid to ask Pip what she means when she "blurts" something out that you don't understand or doesn't sit well with you. Burying your head in the sand about this isn't going to make it go away, as you surely know.
I don't want to breed unnecessary suspicion, but if it were me, I'd be a little sceptical of Pip's true motivations and desires here.