Hi - this is a serious breach of the law - you have effectively come back to no job and are also not being provided with work 'at the appropriate level' which is a recognised breach of both employment law and rules on maternity leave. Also this has a whiff of them having got used to the person who wsa there whilst you were away, and also perhaps that they are a little afraid of undermining her.
On the flip side, though, if your work is seen as at a lower level of responsibility to the other person - it effectively means that her role should be downgraded if more than 50% of her current work is your old work - so it would be in her interests to pass it to you anyway.
Being paid to surf the internet (as I was in my last role when a significant training opportunity that I had scheduled my work around was withdrawn) is soul destroying, an insult and could be easily positioned (as could not providing work at the appropriate level) as evidence of constructive dismissal - I know several people working in a public body who took out a class action (equivalent) on precisely this - withdrawal of work at the appropriate level - who won and carried on working taking on work from a new team brought in elsewhere in the country.
Without knowing much about the work, or personalies etc, its difficult to find a certain route through this - but it certainly sounds like neither of you are being managed well if your manager has a vague plan about the replacement supervising you, but hasn't either put it in place or, it seems, communicated that idea to the replacement - if it is vague and an idea/one interpretation, it might be best to try to set a date when you feel you will have got used to being back and set a date with your replacement to discuss levels of responsibility and boundaries of roles - this should happen anyway so this shouldn't be seen as aggressive, just logical - you could position this as 'hey x(replacement) I'm getting pretty used to being back now, could we meet next week to talk about how i fit in with you, you and I with our boss and all that?., perhaps tuesday?'
If you can pick sensible colleagues that interact with your team, talk it over with them about a) what has been happening b) how they understand current interactions and c) whether they would appreciate guidelines over who to go to for what now there are two staff.
If you position all this as you seeking responsible confirmation and understanding in a way to help your boss get things done a) its difficult to resist b) you can throw your hands up in shock if they accuse you of pshing too hard and say ' no I've been away for a while and I think we should make th most of x(replacement's) higher level skills until I find my feet a bit more. c) HR will support you if it goes a bit weird d) you will look professional and responsible,
At the moment, if your manager is anything like my last, he/she will let you carry on doing nothing, then complain behind your back that you are always doing nothing, so if you get some evidence that you have tried to start a discussion - and others know it, thie will give people the strength to say 'no she hasn't!!'
In the background, though, start making notes about this (EG CUT and paste your entry above and date it), speak to Union or other adviser (this is easy employment law and could probably be covered by a quick half hour chat with an emplloyment lawyer) and just get into the habit of copying pertinent e-mail to a private account, and make notes of discussions - you'll hear this a lot, and it might turn you off, but believe me I have found even the tiny nuance (eg 'x said to me in the corridor that she 'didn't know what I did anyway' - she is my line manager! 3pm 05.06.10) can add to a significant case.
If you and your maternity cover (sorry not replacement!) can generate work, why not propose a strategy to her with your new boss present - and then when you explain strategy divvy up the high and low level tasks immediately you present this (eg I think we could go for 5 new corporate clients with x taking on these three (over 500 employees) and me taking on the rest)...
anyway just some thoughts - but I thik with evidence (eg send the odd e-mail asking for a work schedule or ';just to clarify, do I handle y and x(cover) handles ,,,) you would have a reasonable andd sensible area of concern that should be managed.