How to handle this

[Beansprout30]

I returned from maternity leave earlier in the year, when I returned my colleague had been given my job title, I was never told anything I just found out by seeing her email signature. Small team of three, the newest member is said colleagues best friend who was brought in for extra cover while I was off and kept on permanently. So I'm supervisor and when I returned so was my colleague. I understand this should not have been allowed as it was my job but i never took it further as I'd planned to get pregnant again fairly soon (am now) and didn't want the hassle

This girl has now suddenly using a manager title, nobody has explained anything but she obviously thinks she is in charge now. Friday she emailed me basically pointing the finger that certain things had not been done, there was absolutely no way it was me, I'm almost certain it was her friend as she has a habit of making errors.

So today she sends a snotty email instructing us going forward how we are going to do things. I'm really annoyed that this had nothing to do with me and she clearly thinks she is in charge. I just don't know how to handle it, i have suspicions that there is something going on with her and my boss hence why she's being given promotions etc, fine, I don't care but I don't want to be told what to do by her especially when I've done nothing wrong. I've been doing this job while she was still in school and I've never had any complaints from management or my customers. Do I just ignore her emails and carry on doing things how I've always done? I don't see the point of raising with our manager as I know he will side with her . It's really getting me down and I'm starting to dread going to work