first, hugs to you. . . .
Of course, I don't know what else, if anything is going on, (recently moved? changed school?), but given all things are =, here are some ideas:
1) It's a phase.
But let's not let it become a habit. (see solutions)
2) pre-teen hormones starting to kick in?
May seem early, but that kind of envy and control is related to the sexual politics required to getting the mate your hormones demand.
Of course, in the raw, envy, manipulation, alliances, power and superiority (teen girl paradise) are unpalatable for everyone around, and can spread like cancer. I'd say that due to her age, she simply lacks a layer of sophistication that helps her get what she wants by being nice.
Solutions: (In no particular order)
part 1) You. Seem you are doing pretty well, so this might be redundant:
a) So as to ensure that we disambiguate your position in the family unit, let me remind you that you are the Boss. You are in charge. You are the Queen, The Empress and Goddess on High. Re-affirmation of this is important because this inoculates you against losing it from back-chat, rudeness, unkind words etc. You are above it all
b) Stay flat but kind. When something happens that is designed to get to you, remember that it does not occur to Your Highness that (insert crime here) would ever be appear in the presence of Your Greatness with malicious intent. Any un-worthy behaviour is, of course, due to the need for your firm and kindly guidance to the innocents below Oh Your Great Zen-ness.
Any obvious manipulation (like the spider example you give), laugh/giggle at the ridiculousness of not including the whole family. (see part 2.c)
part 2) Her. Could be also she needs clear boundaries.
a) My take is that she needs compulsory
kindness and sharing training. Do this with her so you are leading by example. This takes the form of doing things like making/getting/giving presents for the others and DH (like baking a cake or cookies) or drawing pictures, or even just taking a toy for nothing in return
. The whole exercise is about forcing her into a position of giving out more than she gets. The "why" should not be explained other than "because that is nice, and we are nice." Practice make perfect, + lots of hugs.
b) In the same vein, maybe she needs some duties ('cos she is so grown up now) so (with you) she can lay the table or something. Here you can remark "I'll give his this because he like that" etc... Don't be afraid to lay it on thick. Killing a problem with kindness works. One way or the other she'll get the message. When she does, don't stop, but integrate it the others (if they are not integrated yet) into the practice of giving.
c) She may need some more planned 1 2 1 mummy time (some 1 2 1 daddy time also a good idea) Important hint:- DON'T go shopping, but rather -do- something, (draw, read or write a story and illustrate, cook, badminton, arm-wrestle...)
good luck. I hope some of this might help you steer right.