You may be great at managing down, but you clearly have a lot to learn about managing up! To be a really great manager, you have to be able to attune to the politics, read what’s really being said to you (‘get off my turf’ in this instance), figure out how to diplomatically get people to do what you want.
Imagine how she’s describing you to her mates: ‘so how is back to work going, Mary?’ ‘Awful, I was only gone for six months, my mat leave blatantly wants my job, has changed how I do things, and now not only spends her time telling me how much better she was at my job and bitching about me to my team, she actually went to Hilary my line manager and complained about me!’
Literally the first thing a friend would say to her is, ‘OMG how awful you can NEVER give her a promotion as she will just stab you in the back.’ You’ve also demonstrated to her line manager that you can’t be constructive and professional in this situation.
Also while you are slagging her off in your team, don’t for a MINUTE think that she is not quite understandably going to be sharing her experience with her peers, likely to be other managers at the level above yours. I’ve lost count of the conversations I’ve had with colleagues coming back from mat-leave about the super-confident-or-arrogant generation below them who Know It All and who are Just So Much Better. We roll our eyes and remind ourselves that in five years time they’ll likely be the ones coming back from second maternity shaking with horror. It is a well known office Thing in our business, just as much as the super cocky assistant who’s previously done three weeks work experience and who Knows Everything, Disdains Admin and REALLY wants to tell everyone how amazing their dissertation was. Don’t be a cliche!!!
She might even have been in HR having a chat about how awful it is and how to manage it. And your team may think you’re brill but they can’t give you a promotion. It’s your boss, your boss’s boss and possibly HR! Whoops!