We are thinking of possibly using my sister's middle name for baby girl's middle name. I've heard you're not suppose to name a baby after a living relative but whatever. I love my sister dearly and the name flows really well. We haven't totally decided yet but if we do decide to do this, do I need to run it by her first? I don't *think* she would pass on the name (she already has one daughter and didn't pass it on) but she is getting married in a few months and is already TTC so should I make sure she hadn't planned on using it for a hypothetical future daughter? Or just tell her "we've decided to name baby xyz..." and see what she thinks?                 
                             
        
Re: Naming etiquette
If my sister chose my name as a middle name for her kids, I would be honored.
I'm Latina and I feel like we do the opposite! Kids are named after everyone and repeated often!
I think your sister would be flattered but if you're asking the question, you might just want to run it by her for peace of mind.
Personally, if you aren't using it as a first name, then I just say go for it. Even if she plans on using it down the road, it's not like your kids would have the exact same name.
I don't think it's weird at all but I get why you're hearing it if it's a cultural thing. We are team green but I know my family would be slightly surprised if we had a boy and didn't either 1) name the baby after my husband (he's already a junior and hates it) or 2) name the baby after my dad. It's for sure a cultural thing on this end. For the record, no first name will be after anyone, it will be a totally fresh name. Middle name, we are completely undecided.
Perhaps ask how she'd feel if that were a possibility and gauge her reaction? But I agree with several other PPs... it is a common name and youre using it as a MN. unless there's some sort of common practice for naming a child after the mother that I don't know about, (like there is for Jr.) I think it's such a sweet idea and your sister should feel honored you are using her name!