I am confused that the new fence is on your sister's boundary, yet the old fence belonged to the neighbour.
So, is the new fence in a slightly different position now? Or is it in exactly the same place?
This sort of thing can cause various problems, as you are no doubt fully aware of - and it's a dangerous game to replace someone else's property and make it your own - no matter how good your intentions are.
If the fence was originally the neighbour's, I feel the neighbour must still have a say regarding the new one.
It's a very common thing nowadays if a neighbour has a shabby old fence, the other people just build a new fence on their land alongside.
Personally, I hate the idea - because you can't work on either fence properly, you waste a bit of your land, you get an area prone to weeds you can't access, and worst of all - the original boundary line is at risk of being forgotten forever.