Please can anyone tell me if I have any rights to see my nieces and nephew. I have fallen out with my sister because she has treated my mum really badly as she is going through chemo. She has stopped us both from seeing the children and we are devastated. My mum is not yet strong enough to take her to court etc, for her rights as a grandparent.

Maybe you should start by trying to make amends with your sister and try and understand why she would treat your mom so badly when she is going through chemo and THEN try and get her to understand her your mom must feel.
I don't think an aunt can go to court for visitation rights of nieces and nephews.

Its still prety rare that grandparents can get visitation, aunt's and uncle's have no standing.

It looks like from what I have read the only true qualified person that could go to court for visitation might be the sister as she is a direct descendant of the mom. I hope that makes sense. Because she is the sister to the children currently residing in another home with the peternal mother then she might be able to request visitation if it is being denied to her. But the best route to take is the one that NOHelp4u recommended and to try to make amends with your sister ( mother of the children ) Try to remember that your all going through a very difficult time.