I see two possibilities:
Things are as you think they are, and she's taking advantage of people, aka work-vampire. This is the more likely scenario, to be honest. If this is the actual situation, try to avoid her, don't boast about being an expert on anything that she might want to take advantage of, and do your job well.
If she asks for your help on something in a way that doesn't have a record, and you're not working on a priority task, try to appear like you're trying to explain to her how to do it, but you're struggling. Make it clear it's a challenge for you to do this task, and having someone watch you do it compounds the issue by causing you to be flustered. In my experience, work vampires usually hate being shown how to do something by someone barely more competent than them.
If she asks you for help while you are working on a priority task, after she leaves, document that she asked for your help and state what task you were working on that was more important. Note that when judging whether your priority task is urgent enough to punt her request for help, you're comparing the tasks. How urgent is it for the department for each task to be performed? If her task isn't something that needs to get done for the department, any work of course takes priority.
She's noted a critical power vacuum, due to your manager's frequent travels, and she's stepping in to help. By your description, in this case, she has many of the skills she needs to do this job, so despite the fact that she does not have the seniority or the official recognition, she's probably not an issue in this case. She's untitled management.
This is the more complicated situation. If your work environment allows for her to distribute work to you in a way that will be officially recognized, she is likely to use that if she's really just filling in the hole as needed. If she does assign tasks to you in a way that you will get recognition for having contributed, don't prioritize it over your other work, but otherwise do it as expediently as feasible. (Basically, since she's not actually in charge, she can't assign priority work. But if she assigns you something that needs to get handled and you're good at handling it, you'll look better for handling it than not. If she assigns you something you're not good at handling, and you can reassign it to a third party who is, hot potato that task to the person who can do it better.)
Not all environments have official tracking of exactly who does what work. Since she's new to the group, even if there is a way to punt things officially, she might not know how to do that, so not making use of it is not a guarantee she's a work-vampire. After she leaves, document any requests she's made of you that you're not authorized to make. If she asked you to help her with something, and you followed the direction for the other case, state in your description of what happened that you attempted to train her on how to do it.
Alternatively, if your environment does have an official way for coworkers to punt work to each other, and she's asking for help on something that you're good at without using the official way to do that, direct her to use the official tool to hand the task over to you. After she leaves, document each time that you direct her in that fashion.
When I say 'document' stuff, I don't necessarily mean writing it down on paper. That can work, but it's not necessarily the best method. You want to record the start and stop times for each event, along with a summary description of the event.
One of the best ways I've encountered to document this sort of thing is to send an email to yourself with all of the particulars, and save it in a folder dedicated to tracking this stuff for her. If you have multiple people that you're tracking this stuff on at once, it's a good idea to make a folder for each person. If your email system deletes older messages, make sure to archive these to permanent storage. If you don't have a dedicated computer at work and you can't save the emails to a USB thumb drive, this isn't a good way for you, so document them via another means. It's important that the documentation method be something permanent.
In pretty much all situations, doing your best to make sure you're not making major mistakes in your own job is in order. A lot of people already do this, and you're probably one of them. But I've known an awful lot of people who were surprised when they got fired, even though they weren't paying particularly close attention to detail. "It was never a problem before, why is it a problem now?" is a common quote. But any time you feel like you're at risk of being somebody's enemy is a good time to double-check on your own work quality and compliance with company policies.
At least four of the best managers I know started out their careers as an untitled manager. They needed a first job out of school, and non-management jobs are far more common than management jobs. None of them studied management, as their understanding of management growing up was 'incompetent'. But they had the talent for the position, and their first or second job was either working for an incompetent manager, a missing manager, or an overworked manager. They saw what needed to be done, and they did it.
All four of the ones I know started their careers like that had some people who accepted their direction, and others who thought they were work vampires. I've heard of other untitled managers, and they were all accused of being work vampires by some people. I just don't know if they were all recognized as helpful.
One person I heard about having worked for a time as an untitled manager was even fired as a work vampire, and then hired back two weeks later, once the boss realized that everything fell apart without him. (This person is not one of my four mentioned above because I haven't worked with him professionally, so I can't say for certain he's a good manager. Also, he was in the industry for a decade before he had this experience; it was not the start of his career.)
Identifying them is not always easy. Some key differences to look for:
Work vampires almost always want credit for doing the work. Untitled managers tend to want credit for expediting the work.
In some environments, untitled managers will need to claim credit for some work others do for them, and in environments where work is only credited to the person assigned, they may have no readily available option to give credit appropriately.
Some work vampires couch their requests for assistance as requests to be trained how to do this task. The training will take longer than just doing the task. But they won't learn, and they'll be back for more later. It'll go just as slowly the next time.
Untitled managers are much more interested in passing the work off to someone who will do it, and getting on to some other task.
A team with a work vampire will be less productive overall than they were without the person. After we fired the first obvious work vampire I encountered in my career, the three people most impacted at least doubled their productivity and everyone on the team was measurably more productive.
A team with an untitled manager will overall be more productive than without the added guidance.
Either sort of employee may be willing to work broker, if there's someone else who can do the other task better. "I'll do this thing for me if you take this other thing I need to do off my plate." If it's a work-vampire, the traded task will never get done. An untitled manager will make sure it gets handled by someone who can do it, if it needs to get done.
