Do you tell your friend when shes in the wrong?

Say your friend isnt actually asking you if shes right or wrong, shes just venting/complaining, but you actually feel shes in the wrong. Do you nicely tell her that, or just listen? My friend has always had a difficult relationship with her sister. Well lately in her stories, Im actually siding with her sister. Example most recently shes mad that her sister had her c section on my friend's son's bday and has stopped speaking to her, saying she should have picked a different date so their kids wont share a bday. My friend and her sister live several states away from each other and hardly see each other anyway, maybe once a year, sometimes every 2 years. I actually live close to her sister and agree that her sister was in a pickle because the local hospital where her sister birthed (where I birthed too) is very tiny and you dont actually get much choice as to your c section date, so I doubt she chose that not-ideal date intentionally. And since theyre so distanced and dont do family birthday parties anyway, it feels like my friend is being ridiculous and for my friend to no longer speak to her sister over this. And there are other examples where she vents and my head tilts. My husband says if I tell her that shes in the wrong, it's pointless because then she will just he mad at me and still hate her sister anyway lol. Im curious what you would do or how you handle situations like this?