I am torn. Like @FilthyforFirth I find it unusual for someone who has nieces and nephews to not want to be called auntie or uncle. I am auntie Patti to mine and DH's nieces and nephews, and my husband is their uncle, (and they all call him Uncle Steve...)
I also call the siblings of my parents 'auntie' or 'uncle,' and have never called them by their first name.
As I said, that is just ME, and I appreciate that others have different opinions, and they are entitled to them...
Despite my views and experiences, @Boytrio your sister is entitled to not want to be called auntie.
It could be even that she does wants kids and can't have them, and being called auntie stings her, as it reminds her that she can't have them. However, if this is NOT the case; as previous posters have said, don't assume it's her husband stopping them having kids, because you don't know that.
I haven't clicked YABU or YANBU, because, as I said, I am on the fence... However, I do think you are being given a weirdly hard time on here. Why people have to be so harsh, and mean, and frankly quite nasty, just baffles me. 
nb, Steve isn't my husband's name...