There might be another way of looking at this. This lady has been home for a few years and maybe she isn't enjoying it as much as she thought or the kids are getting to kinder/school age and she is thinking about returning to the work force. She has heard about your news and thinks there is an opportunity to update her work skills so that any future employers won't dismiss her as being out of the work force for too long. As it was her job before it was yours and she knows the job it won't be as hard as learning a new job and so she can balance her family and working again more easily. She will be aware it is only for the term of your maternity leave and they will have to find you a position on your return. You won't be out of a job.

Putting the shoe on the other foot if this happens to you and after having your beautiful little boy you can't leave him and stay home for a few years and then decide you want to return to work and there was a short term opportunity in a job you loved and knew, where you knew you had been highly respected you would hope your former employer would be happy to have you back as long as everyone knew where they stood.

I don't think you have to worry about it.