A close family member has 'put dibs' on a names three times. First was years before either of us was pregnant and I just made a mental note it was out of bounds as it was her favourite name, and she did use it years later.
Second time was when I was pregnant (she wasn't) and wanted to make sure I didn't use another of her favourite names (it was OK as it wasn't on my list anyway, but I was a bit
that it had happened again).
Third time we're both pregnant, and the 'reserved' name was one that she was aware was on my list from the last pregnancy. I was a bit miffed. We've both now settled on different names so again it's fine. If I have another baby I'm considering getting in there first and bagging one of my own though! 
I think as a rule of thumb, if you are both already pregnant and there's one name only that you'd want to use, then it's OK to say 'listen, there is only one name we love and I'm worried that it's going to get used - can I see if this is on your list?'. If not, then I think any name is fair game and we all have to take our chances if someone uses it first. I can see the IVF situation is a tough one though as if she's been TTC for years she must have seen a lot of names she likes bestowed on other babies.
I certainly don't think anyone gets to reserve a few names just in case, especially if they already know they are on your list...you'd be kind to compromise on this one but any more is a proper cheek...